So traveling with spouse and child (8yrs) from USA through LHR to destination DEL. We have a layover of 6 hrs 25 mins on way over (arrive T3, depart T5) and on return 7 hrs 40 mins (arrive T5, depart T5). US citizens. On return flight i opted out of shorter layover of 1 hr 40 mins unsure if that would cause trouble but maybe i should have opted for that. Some questions to navigate LHR and consider options for long layovers esp with child.
1) Is there enough time to leave the airport and return back and grab some quick coffee/lunch outside, especially on the longer layover on return. I could apply for the evisa for London as still have 10 days left... I am not sure how the immigration process is. Haven't been to UK in literally ages so need simplified things but would be nice to get child out for a real experience eating outside of airport if possible.
2) If leaving isn't advisable and even for shorter layover... Is there an onsite hotel (airside/landside)? for some rest? Would need visa if landside obviously? Is that advisable?
3) Any lounges recommended? Do i really need the extended lounge pass if I purchase it for 3-6 hrs or is part of that spent clearing some sort of immigration process, lines etc? Last time I transited LHR, it seems like going through hoops to go from one flight to another and that was with wheelchair assist. I can now walk but extensive walking I can't do. I have lots of time to take it slow though. I don't think I have any access through any point system. Flying WT+ but know that doesn't give any lounge access.
4) are paid lounges in T3 better than T5?
5) Is going from T3 to T5 fairly straightforward?

A bit stressed as this is child's first international travel.