1) In general I've found that T-Mobile's free internati
nal service works fine if you download offline maps and any language translations that you might need. I travel to a lot of different countries and almost never bother with a local SIM/eSIM if I have T-Mobile service. I will say I don't use Instagram / Faceblech / etc. For Whatsapp, Uber, email I find it to be just fine. It's not as fast as at home, but it's perfectly serviceable.
5) You do need to pre-book the Holiday Inn express in the terminal. Upon arrival, you must to find the hotel's desk (there are signs with the info) and someone from the desk will escort you to the hotel. You can't get to the hotel by yourself. Also, the
hotel's web site has this warning:
Any guest flying out from Terminal 3 within 24 hours of arrival at airport can stay in Domestic Wing. For International Wing, guest must have boarding pass for next international flight and their baggage must be checked through till final destination. Passengers arriving from Domestic or International flight at T3 may reach out to our help desk near Domestic transfers for any assistance required.