Having the trailer level is best. Having some amount of rear tow vehicle sag is OK. I would allow a little trailer tongue drop is you can't find a tow bar that's "just right".
If your tow vehicle wanders like puppy and is hard to keep in a straight line (and the trailer isn't swaying) then you need to get a WD hitch setup to get the weight distributed to the front end of your tow vehicle. There are a lot of variables.
After installing your safety chains, the breakaway switch cable must be connected to the hitch chain loop, independent of the safety chains, never intertwined in the chains and free from obstruction. If it's just too darn long, make it shorter, but not so short it could yank out in a turn or going over a large dip int he road. Leave a little extra.
If your safety chains are dragging or really close to the road, get some links cut out. Don't make them too short, they should have some droop.
For the breakaway torpedo, if you have catastrophe while towing and the chains break, the coupling breaks, the tow ball breaks, whichever, if the breakaway plastic torpedo is pulled , it will connect your trailer brakes directly to your 12 volt battery and slow the trailer down so doesn't run away into someones house.