Why does my MacBook wont start up anymore.

Problem with MacBook starting up with battery which requires maintenance. MacBook kept on working until after two weeks after the first warning it wont power up again after a few days

MacBook involved is an Air series mid 2012-2017.

So what we tried.

-MacBook wont power on

-MacBook also wont charge

-However, a bit later we found out the MacBook does seem to charge when first connecting MagSafe to similar other (working) MacBook and then connecting the MagSafe to MacBook which won’t start up

-MagSafe then lights up; even says it charging the battery

-When clicking the on button after a few hours of ‘charging’ with green light on MagSafe, the MacBook started up ones with sound, but not video other than black screen and then no response at all anymore

-trying some more things with MacBook, e.i. Switching battery of working MacBook in not working MacBook

-Then MacBook started up again (now for 2nd time since ‘dead symptoms’). This time even with screen working!, system was initializing, but crashed (shut of) a few seconds after logging in

-after that no more response, no start up sound, no screen life signs. Tried SMC reset and other commands against but nothing seems to work.

Ordered a new battery but since the working battery from the other MacBook also doesn’t seem to help, would this be the solution?

Also read something about the trackpad connector being a reason for the MacBook not starting up again after the first initialization. Would it be this or something else ?

Any help is appreciated, thanks in advance !