Can this charge an Apple mini iPad?

No it can’t.

it will stop charging my phone once the battery is 80%, what should i do?

I don't know, mine works well.

will it stop charging automatically if my phone is fully charged?

Don't know for sure, but I believe so. Have had mine for about a year with iphoneX and everything is still working fine.

Will it charge while the phone is turned off?

I think it will, but you have to make sure that it have been placed on the charging area.

which ac adapter should i use with this?

the one comes with your phone or any others that can accommodate a USB port, except iPhone wall charger

My samsung s9 charges only 7% in 10 mins with samsung's original fast charge adapter. charges a little bit slowly?

it takes about 2.5 hours for my s8 fully charged

Will this mess up the iPhone X battery life?

nope, mine work with my iphone 8 with no problem

it doesn't come with an wall charger?

yes, I used an old note 8 charger cube to plug into the wall and it works perfectly.

it supposed to be overheated?

no, this one with overheated protection, would stop charging if overheated.

will ths work with iphone 7 plus?

No, for iPhone X, 8 and 8 plus only as described.