Scroll to the end of my extremely long reply, sorry for that, For my honest thoughts on your post. I really took your situation to heart.
As a father of a (happy birthday) 6 year old, your post throws me back to the days of mentions and my heart hurts as I KNOW the feeling you are having. Our daughter was born with several orthopedic concerns , which we were aware of as ultrasounds revealed. Tibia Hememelia (bone deformation between the knee and ankle, missing bone and severe curviture) in her rgith leg, Bi lateral club feet, lower spine deformation( curved spine? scoliosis?) A few Fused ribs, and teh leg with the Tibia H- the Hip was deformed and never formed so she now has a permanent (for now?) Dislocated R-Hip. Sorry im not a dr MOM knows the exact names if i messed them up.
OK saying that we were already planning thinking etc how this is going to work etc Life challenges. But we had a GREAT orthopaedic Surgeon and team working with us to prepare. SO the BIG day! here she comes everything was ok standard , min jaundice got the suntan lamp for a few days

she was tongue tied and couldn't breast feed so it was clipped. YAY we are home LOVE her. After a few visits to Children Hospital (was a very frequent event every other day) Tibia hemimilia is apparently quite rare so dr's were very curious to see and we were working on casting bi lateral correction etc,
One day an intern caught what appeared to be a heart murmer....This is where things got HARD. A few checks and Here we go Our 5 month old baby Now revealed she (basically) 1.5 lungs and the right side lung wasnt attached to the heart and was causing hypertension.
NO dark details here the next 4-6 months were extremely trying as we where being transitioned back and forth from Ottawa CHEO to Toronto SICK KIDS. staying at Ronald McDonald house sleeping Hospital waiting room couches, the whole experience put me into auto pilot. I wont lie I was scared but stayed strong for mom and baby. SO.. several heart surgeries thorocodomy and cardiac procedures valve replacements and a ton more things i cant even fathom to remember (moms Know)

over months we finally fixed the plumbing

giggle.
PHEW! then after her final procedure they went to (extrabate?) pull the assisted breathing tube from her lil body and her vocal cords got paralyzed. so few more weeks of monitoring and such finally was decided that we could transfer back to OTTAWA CHEO . SO now we were dealing with HOW can the baby Breathe? tracheotomy...Mom knows

was the fear. Counted our blessings when one day in the hospital ENT decided to try one more time to pull the tube........... As the tube left contact The LITTLE.. BaaahhWaaahhhh Mom and DAD cried. She would be fine / partial paralysis but monitored and honestly i think we are in the clear as she can scream real loud now lol XD.
OK.. Now back to the legs and ortho....as OF course we stopped all procedures until the plumbing was figured out then we will look at the frame

skeleton

so she had the procedure that portion of her right leg amputated and went through that process also castings etc and we are still going through the restrictions / limitations of it and other factors its just our normal for us now but does get hard at times. OK so saying all this all before she was 1? maybe 2. so saying this she still has cardiac appointments (life) othopedic steps as of now just monitoring growths and functions etc. its just the the way it goes. But she is an amazing lil girl.
(the reason i felt all this was important to say was everything she has gone through her development took back seats (delayed) and once all health issues were adressed she progressed over time and caught up with some of the concerns they thought may exisit or cause concern......dont at this time.)
Areas of Concern:
x Hand flapping a lot, no matter the emotional state (motor function) typically learning control of motor skills)or is just fun to flail
x Tenses upper body, holds mouth open or does a shake in excitement (seizures ruled out) ..
Ok this must be a kid thing my lil one used to stare off into space as well I did get concerned and asked and check nothing....its a tad freaky for me anyways..but now we lol about it wife says she is looking at (spirits) Her DAD my DAD and MOM etc..I will be honest I seen a few times she would watch something come down the stairs and follow it with her eys and head and nothing there. I will leave it at that But it isnt that uncommon IT is spooky to see...
x Does not seem to understand much, if you ask where is mama or dadda :
lol have that problem NOW as well as at that age...I feel its not that they dont hear it its them processing the proceess.. it will develop repetition. Comprehension is a whole new game.
x Receptive language is my primary concern x Mouths toys all the time, plays appropriately only 10% of the time : mouthing words is good our lil one used to look in a floor mirror and do all sorts of weird things including SHAPING mouth. Receptive LANGUAGE? If you call them do they respond? even if they just turn to look etc. where is mom where is dad? not to sure we did that game lol but she got it at some point she acknowledged the voices. pretty sure she was older than 14 months. regarding appropriate playing? IDK what that means lol again it is at the Development stage and kids are at all different levels being behind a curve doesnt mean anything is wrong stuff just happens youll just have to take my word that I felt all the same things your are kind of feeling and now that i look back smile.... Only thing i wish is I had more time to enjoy that time with her instead of analizing ...wait does that mean this does this mean that ....youll go crazy HAVE FUN and it will just happen and you will be in the same bout in the future giving advice opinions and shared feelings to others.
x Likes to touch children, not sure if this is a pro or con. Seems very interested in them. Loves starring at them : yes its funny just simple curiosity IMHO our lil one is 6 and she still analyzes and watches everything i can see the wheels turning so to speak.
But everything you have listed ....(FOLLOW ADVICE FROM medical) to me seems absolutely normal behaviors and i have experienced most if not all at some stage of my daughters life.
MY point here is at 14 months they are still developmental stages (DR's and NURSES and Specialists follow growth charts / development charts Milestones etc...if they are saying NOT WORRIED it probably because its to early to determine FACTUAL concerns IF ANY. "OT" Love these people they are your window and guide if there are any concerns. My daughter still has an OT external and an OT in the school she is in. Dr's Nurses, OTs and everything and everyone ....USE as informative TOOLS.
"..yet all of my internet searches and books I've read say otherwise." That sir is a rabbit hole i wouldn't recommend to take as sound advice unfortunately its the world we live in now. trust me everything my daughter had and has i goggled and started searching sent me spiraling into the rabbit hole (Alice N wonderland)

That did nothing but increase my heart rate and drain my energy. Saying this EVEN WHAT I WRITE should be taken as exposure opinion to concerns and how it was/is applicable to my situation...life...nothing more nothing less. extract information that helps, and toss the rest away build your own opinion after you get all the choices and information. this is just how i approach things naturally in life. Just Breathe DAD.
From 1 Dad to another:
Try to relax and enjoy this time ...I know its hard and scary and challenging as we ALL want nothing but the best for our children and families. I wish i had focused more on HER than I did worrying about the WHAT ifs, is that normal etc along with HUGE life choices we had to make. Really it will suck the life out of you.
Take the issues 1 at a time....but give them time and TRY, i say try cause if you are like me things run in my head all the time, try and just enjoy the very small things simple smiles simple giggles funny feeding moments...funny Diaper(moments) maybe not funny as they happen but they are later lol. All in all sir try and enjoy the time as it goes by FAST!
BE dad and everything else just sort of lines up.
ALL in ALL at the start and end of every day and while they dream make sure they know DAD is and will always be there.
Have a great Day and weekend.