Here's my recollection (from memory and we didn't have anyone with mobility issues when we were there, but I'm fairly aware of mobility challenges in general because I have many times travelled with someone with a walker or issues with stairs).
* First - are those floorplans ever correct, my guess is they are representative of a general layout, I don't think I've seen one correctly represent a mobility unit, for example - my money says they use the same floorplan for all unit types and cannot be trusted, especially for a mobility unit
* Inn on the Harbor parking is very limited.
- parking under the building (parking garageish scenario) gives you access to an entryway that I'm pretty sure requires you to go up a step or two to enter - and then if I am recalling correctly, up 1/2 flight to get to an elevator (the elevator in the lobby of the resort).
- there is a small parking lot to the south of the building / lobby entrance. if you were to park there I think you can get into the building lobby to register and/or use the elevator without any stairs to manuever (there is very limited parking in that lot, but hopefully with a handicap pass you'd be able to find something there)
* So yes, there is an elevator that then takes you to all floors - you can likely figure out how to get to it (the lobby entrance/elevator) without encountering any stairs
* There is a bar/restaurant on the 1st floor (I can't remember if it's accessible from the lobby floor, how to get to it escapes me. If you walk through the bar/restaurant it'll take you to the wharf. Very cute area. However, I'm sure there are a few steps down to the dock. I do not recall wheelchair type access to it (but I wasn't looking).
* The wharf area is very cute - there were baby sharks living under the dock (having a 5 year old shark fanatic with us we spent a lot of time there)
* There is a beach area to the north (between Inn on the Harbor and the Newport Onshore), there is a pool at Newport Onshore but I don't think you are supposed to use it if staying at Inn on the Harbor (even though the share some management duties (like overnight checkins are done at the Inn on the Harbor for both resorts)). We had family at both resorts and it was easy to walk between the 2 resorts, but doubtful it would have been with a wheelchair or walker
* The rooms - we did not have any accessible room - but surely they would have a walkin shower in an accessible room? The rooms are that funky layout where you walk through the bedroom, past the bathroom, to the living area. I think they are all that way?
* Really nice laundry room, game room
* Rooftop is amazing - I think the elevator takes you all the way up but wouldn't swear to it (ask)
I would call back and ask for a manager / someone who really knows and ask very specifically about the accessibility of it (if I had someone with a walker or wheelchair or even bad knees).
If you go to the cliff walk/mansion area, there is a free trolley that runs through that area.
Usually you need to specifically book an accessible unit. If I needed one, I would book accessible and then call and let know I really need the accessible. If there isn't an accessible unit available, and you book a standard room, still call and let them know you need an accessible unit (but it won't be guaranteed). Most (at least many) of the resorts will have things to assist if you end up in a standard room (like a shower chair or raised toilet seat with handles).