Your math on the SPG transfer is spot on. 20k=25k so 100k=125k (obviously pretty simply)

It's basically a 1:1.25 ratio (or a 2.4:1 ratio expressed in Marriott terms)
So there's 2 schools of thought on Marriott Nights & Flights in my opinion. (1) People who have a 7 night hotel stay in mind and are getting the airline points as a bonus AND (2) People who are doing it purely for the miles (and the nights are a throw-in bonus of sorts). The People in #1 or doing it as Marriott intended. Let me analyze both here (for option 1 we'll use your cat 6 as our example):
(1) People with a 7 night hotel stay in mind
- I am planning a 7 night stay at a category 6 property. Without Nights & Flights this would cost me 30k points per night.
- Since I get the 5th night free, 7 nights costs 180,000 Marriott points (30kx6 + 1 free)
- Now I can also look at a Nights & Flights...and I can transfer 50k, 70k, 100k or 120k add'l Marriott points at a 1:1 ratio to most of my favorite airlines
- So 180k (for nights) + 120k (for flights) gets me 7 nights at a hotel AND 120k air miles/points
(2) People with no stay in mind
- I just want the best redemption rate possible - who cares if I ever even use the nights (hopefully I do)
- I can transfer 270k Marriott points and get 120k airline miles/points at 2.25:1 (compare that the earlier 2.4:1 and it even beats that)
- Oh yay, I also get 7 nights at a Cat1-5 property. If I only use 4 of those nights, who cares...I did it solely for the miles.
Option 2 isn't bad and as you can see at the right level even beats the SPG transfers. HOWEVER if you're an option 1 person you are using this program as intended and you have won the day my friend!!

If somebody is an option 1 person, the more efficiently they use the free nights will increase their actual savings/return.
Obviously the United deal is even better since Marriott has that special deal with them that gives you a 10% bump on miles when you use these transfer programs.
I do not know the answer on booking ahead of time but my guess is YES you could do it. I would book it now so it's locked in and then see what happens from there

Let me know if you have any other questions.