Generally Hibiscus can take a heavy pruning and will quickly sprout new shoots all over. If you're in a cold climate you need to do that during a suitable warmer part of the year. You can then manage the new shoots into a shape you like.

you can cut it by a third, but look for buds , it's better to trim above the leaves that have a few beginner buds. generally it's better to let the trunk mature a bit before you trim a lot.

the wooded trunk will branch usually better. but when the plant is very young, you probably won't have enough wooded trunk to work with. pinch it for now just to slow it down, so it does not get too leggy before you can put it outside. I assume you will put it outside? it won't do well indoors.

yes - if you take the very tip of the main growth stem off, you will encourage the plant to focus on producing more branches lower down. all you need to do is pinch off the very tip of the growth with your finger - and that will stop the straight up growth and encourage more side growth. of course you could also take off say 6 in from the top and try to root a cutting to get another plant :) cut just above a growth node.