Yes. You can. It is very normal, but will take some time if you want to do it yourself.
NO.
1964 to 1987 El Caminos swap to their A-Body sister cars and vice versa
If the cubic inch is the same the motors should swap out ok. You will have to change some parts but most Chevy motors will swap.
Yes you can.
Yes, That is a direct swap. It's all the same.
Yes
YES, That would be a direct bolt in with NO problems. EVEN SWAP
Yes.
no
the g20 should be a 350 and I believe the caprice is a 305 you can swap the engines but you will also probably have to swap the intake and exhaust manifolds
Everything
yes just have to swap the computer, wring harness and sensors