First off, Knott's has on paper what sounds like a similar system, but the reality is completely different.
DL uses kiosks to get the return times. At Knott's, you have to go to each attraction and most of the wait times just to get to someone to give you the return time is longer than the wait times, but if you can't do stairs, then you have no other choice.
Knott's has a 30 minute (last I heard) window to come back within and if you don't, then you have to get a new wait time.
When you return, you have to wait again to get to where you can enter (usually the same wait you waited to get the return time).
Disneyland, you go to the Kiosk, tell them what you want to ride next and they will write the return time and you can go anytime after that return time, but you can't have more than one return time at a time.
That being said, you can use FP and if a wheelchair is used, wheelchair return time cards (these have a 1 hour return time window) as well.
I have been on a very busy day and using the
DAS does work well, the only thing to keep in mind is if it is that busy, getting around the parks may be more difficult, but with proper planning, it's still doable.
I would not expect 2/14 to be a 10/10 day, but I would expect it be fairly busy. If you can get there at park opening, it won't be as bad though. In fact it will probably be fairly light until about 2:00 is when you will start noticing the crowds, then be quite crowded from about 4:30 on.