He really does come back to the village after four month according to the web novel and no there is no time difference since the lab was in the same world as the village was.
Time to build some Gundam's and start taking over the world. Muwahahaha.... Yeah right, I bet the first thing he goes off to do is find rice and/or make soy sauce.
Only the entrance was collapsed. He can still enter it from the transporter and just get rid of the wall he created. (which he does much later on when he finally chooses to return to the village)
He doesn't go full mad scientist just yet, first he's going to travel and gain a few more girls for his harem.
It's only once the threat of war looms on the horizon that he takes off the kid gloves and stops restraining himself so much. We're not quite are "orbital death satellites" yet, but 'advanced' at least.