That's quite a list of questions!
The only sure way to tell how deep the topsoil is is to dig a test hole, be it with a shovel, back hoe or auger. Is the topsoil yours after the fact? Around here, topsoil is more valuable than engineered fill, so selling the leftovers makes up for what you have to spend on fill.
Several questions about compaction. 3 or 4 inch minus bank run at 8" lifts. Rent a walk-behind sheepfoot roller and roll the heck out of it. I would imagine no one is going to pay for a tester? So no real way of telling how tight it's getting. After a couple lifts, find the heaviest truck you can and see if it pumps (moves) while wheel-rolling it.
You can never have topsoil too deep. That being said, deeper than 4 to 6 inches is most likely 'undisturbed native soil' which means that due to time and gravity, it's as tight as it's going to get.
Spreading gravel is hard work. Some places have rock chucker trucks, and they mostly do a nice job. Sometimes the local high school football coach needs a nice workout for his team:whistling Use inch minus, crushed not screened. Bigger the rocks, harder to spread. 4" minimum, more is better in freeze & thaw conditions.
Whether you use a string or a laser makes no difference, but the more you check, the better off you'll be. I don't have my calculator handy, but 1 cubic yard of concrete will do a 9' square 4" deep. Every quarter inch thicker adds a couple wheelbarrows more mud.