umh this makes me think i was isekaid already but not in the traditional way, i think he got isekaid when the majin god showed up in ch 0, his power was untrained at that point, and he won but got isekaid, then he had a whole adventure on the other side and finally managed to come back, but being in that world soft-reseted the memories of the ppl he knew about him, thats why he was so happy about his 2 friends remembering him, and thats also why the 3rd friend recognized she knew him after a while and also why he talks about protecting this ''normal'' high school life, coz he already lived a whole ass isekai life.
Now about his powers, it looks like they dont work according to the system of his native world so he couldnt use them properly, maybe its like he has SP (like in persona 3) when everyone else has MP, but maybe in the isekai world SP is the regular system so he could fight properly there, now upon returning to his world he cant use his 999 SP (coz of course he would be high lvl at this point), so the grandpa decided to have him learn ''Sendou'' that probably uses SP, they said hes on par with someone who trained hundred of years and this is probably coz of the whole isekai leveling, the granpa said that when he uses his power ppl forget about him, so sendou is the better option but what is his power? maybe its a skill that turns memories about him into power kinda thing?
And finally the stuff in this chapter, about that girl that was probably a princess and him remembering his sword, finally his mom....umh maybe she went to the other world to search for him? who knows but anyway this was some crack ass theory, ty for reading