Keisha, I've always reserved Mythos the same day I've wanted to eat there. If you plan on getting there when they open (11:30), you don't even need a ressie. Literally. The dining booths at Citywalk and IOA wouldn't even give me one, lol.
Regarding your concierge question, we just returned and had stayed at HRH. The parks/hotels were packed full of a Kraft convention and tons of school/cheerleading groups. I thought for sure dining would be hard, so I called the "dining hotline" on the phones in the room. It dumped me over to Star Services, which is fine. I think almost everything dumps you to Star Services now -- we had a pest control issue in our room and called housekeeping; Star Services answered.
Anyway, we thought it would be best to make some dinning ressies through the hotline, and it was a giant headache and really unnecessary. Our ressie at Mythos never even got to Mythos. We checked in at noon, and they had no ressie for us (thankfully, there was plenty of seating and no wait). I called to book a ressie at NBA on Friday night and was told it wasn't necessary because our room key would give us next available table. I thought, cool. Well, while waiting for my group to get ready, I read the little "book" that's in the room about all the hotel amenities and how your room key does act as a ressie, EXCEPT on Fridays, Saturdays, holidays and special events. Ugh!!!
I ended up calling 411 from my cell phone to get NBA's number, called them direct and just booked a ressie myself. Glad I did, too, because there was some major basketball games on and the wait was unreal! The lady on the phone did confirm that our room key would not have acted as a ressie on a busy weekend night for dinner. This was all on the same day, BTW.
The only place I've ever really stressed about a ressie is at Tchoup Chop. I called a whole month in advance
Just my experience with letting the folks at HRH handle our ressies. IMHO, it's totally unnecessary, and in some cases, if it's crazy busy and an absolute must-do (and you know in advance that's where you want to go), you're better to just call the restaurant direct when it gets real close to your trip.