The Heli Coil is not pressed in.  First the existing threads are drilled out with a 1/4" diameter drill.  Then a special tap is used to cut new threads in the aluminum, then a tool is used to thread the insert in place.  The insert is supposed to lock itself into the hole - I use a very small amount of threadlocker just to be sure.

Since the engine has run so little - the gaskets may not be stuck to anything and you might get away with using them again if they are not damaged in any way.  If the rubber sealing washers were new.....I would certainly use them again.

If you find somebody local that can install the heli-coil - I can sell you one of the 18mm long inserts to use.

Your technique needs refined.
Bolts don't strip on their own.
Torque in stages. Start with a ratchet, firm up every bolt, run the pattern, back and forth, pulling it together square and even. Don't shoot for the high end of the torque range.
Feel of the bolt. It will change how it feels when the
Clamping Force gets up around where it needs to be.
It's easier to fix a leak
Than wrecked threads.