e-sim. Split people within rooms/traveling groups with a mix of Ubigi and whatever else they want not on Docomo https://www.flyertalk.com/forum/37306190-post812.html. That way everyone will have their own data in case they get split up. People who use different amounts of data can get whatever fits their needs.

Fairly sure AI would also give you the pros and cons if you asked, but experience says plan for redundancy and accidentally getting split up.

The other questions - since you're asking for a trip you aren't going on - my unsolicited opinion is to let them do the rest of the planning. We've covered ways to get into Tokyo from HND to Asakusa. "With luggage" is subjective depending on how large luggage is and how mobile people are. If they can't carry a checked bag up 3 flights of stairs, know that they should look for ways around that either by researching for JR or metro exit with elevator or escalator (most exits are stairs only) or an alternative like the ones you've mentioned.

Honestly, if they are comfortable, AI isn't a bad choice after perusing japan-guide, especially since one family hasn't visited so they won't have a comparison. It's not like you'll be on the trip to help answer questions on the spot. It's hard enough planning around interests, mobility, dietary needs, and stamina for one group. Combining can be even more difficult. I would say someone should either pay for an account with ChatGPT or Gemini as they're better than the free models. Also ask AI to validate and double check what it suggests against opening hours, distance, planning around realistic crowds (and waits), and any other specific needs.

Some people stress about doing Japan "wrong" or feel obligated to pack their schedule. There's ways to optimize but as long as they're curious and they know what kind of travelers they are, then I'm sure they'll have fun. Just watch what locals do then be slightly more polite as travelers are guests. Oh, and avoid planning around influencer TikTok/instagram recommended eateries and "must see!" but "meh-to-them" attractions as they'll suck up way too much time standing in line.