A galaxy is still a galaxy. Something holds it together.

Ageed. There's gravity. But even gravity doesn't act instantaneously.
Like some things, it's effect propogates only at light speed. A change
in Andromeda wouldn't be felt here for over 2.5 million years !

A car is just a collection of atoms. Yet cars collide.

Again agree, BUT colliding cars are unlike merging galaxies in that
there will be essentially no collisions in the latter.

Never mind about light speed. ...

What ?! Haven't you heard of Einstein's relativities ? Absolutely
nothing has ever been observed to exceed light speed, in all astronomy
and even in the big accellorators ( SP ? ), ( Cern ?, Hadron ? ).