Hi, ok: some comments responding to your second note.
First, I would not bother with the disabled key. Its biggest use is for un-attended WC facilities, which sounds largely irrelevant for your partner.
Second, that hotel is very centrally located, which should open up a lot of opportunities for your partner to explore the central areas of the city rather straightforwardly. I would double-check explicitly with the hotel about steps either at the entrance or leading to the elevator/room, although I would rate that as rather unlikely. Oh, and you probably already thought of this, but you may want to check into the shower type at the hotel--it is not uncommon in DE to have only a rather small and in no way accessible stand-up shower, but this is a pretty fancy hotel, so the facilities are probably better.
The main conceptual thing to be aware of when navigating in DE with a wheelchair is that there is nothing like Title III of ADA here, so there is really no 'guarantee' that public accommodations are accessible. That said, there are many spaces that are accessible (and a growing number--I remember when my sister was a child, how much more limited access was). And, that said, a lot of the public transportation infrastructure here (to name one example) is really new, and is therefore more easily accessible than what one finds in the US. But in general, one needs to expect many more barriers to mobility in public accommodations here, alongside both public and private efforts to improve the situation.
About urban centres: curb cuts are pretty universal, but sadly really brutal rough cobblestones are pretty common too (though in no way universal). But, that's something to watch out for, if your partner is not accustomed to them-- front wheels can get caught easily. Also, because of the lack of public accommodations rules, it is really quite common for businesses etc to have raised thresholds or un-ramped steps or so on. One fairly good resource to use is wheelmap.org. It is not perfect by any means (crowdsourced data for the most part), but should give a bit of a sense. Be careful with using Google StreetView for up-to-date information, though: Because of privacy rights concerns, Google has not been allowed to take new streetview images for over 10 years in DE, so any that you happen to find will be quite dated at this point.
Finally, is there a type of thing that your partner is interested in seeing (i.e., museums, nature, streetlife, whatever)? I actually think that, given the central location of the hotel, there's a really reasonable opportunity to access a fair number of things, and not be stuck with the one or two places nearby that happen to be accessible...