"My grandfather kidnapped me and brought me into another world to work on his farm"
It's a good thing MC's didn't give up his dream of being a farmer and had no lingering attachments to Japan or he'd be hell of pissed right now.
It's also a good thing that tactical nuke envelope didn't accidentally isekai everyone else in the office too (jeez grandpa, give a little warning next time to open it somewhere alone).
Why didn't the grandpa just leave him all the IRL things to him too instead of to the shitty relatives? Was he stupid?
Because grandpa wanted to genderswap MC for the lulz.
In an alternate universe, it would of been funny if he actually did give him the IRL house and tricked his shitty relatives into opening the letter and trapping them on his farm in the other world forever.
Is that all the explanation for the gender switch situation or why he's been rejuvenated we'll ever get?
Looks like it. I don't mind the random genderswap when MC crosses other dimensions. Kinda wish we had some kind of explanation for why he regressed to a 10 year old rather than gloss over it though.