The pins are heavy to help match the weight of factory pistons. Replace with lighter ones if you are going to re balance.
In theory you wouldn't have to add heavy
The gear debris will get past the pickup screen and also the pump. I have pictures of the block and crank oil passages completely clogged of gear and caused a spun bearing. Don't take the easy way out. Exactly the same issue when blowing a stock head gasket.
More important is did you completely disassemble to clean all the gear debris?
That will cause a spun bearing. Most people don't.
The stock pump is fine.
Yes it is i've done it a few times.
You need a yolk for the trans to keep it from leaking all the fluid out.
You also need a leveler or "lazy susan" to tilt it up and out.
I'd make sure the trans lines go in at the same time.
Wait until its in to install the trq converter bolts under the car.
Another thing to consider that is often over looked, will you have the correct piston to wall clearance after you hone?
If it has high mileage and a lot of wear, it might take a thou or more to clean up and that might put you out of spec....