- How important is it to trim back the branches where the wires will pass, so no wires rub on limbs? I am not looking to amputate the trees unless I really have to "
Use insulated wire and you don't need to worry. I've used bare wire against trees without trouble, but if the high voltage points of the antenna rub against branches you might have SWR stability trouble and possible smoke/fire with high power, plus it could cause noise at least in theory.
"- Any suggestions for building flexibility into the antenna's connections to the trees? "
I used window sash weights on a rope through a pulley to provide strain relief on my whippiest tree supports. That worked well.
Counterweights are good because they provide a constant pull no matter the displacement of the tree. I've used garage door springs on some antennas too but they pull harder in stronger winds, so you need to be careful.
Some people use bungees but in my experience bungee rubber doesn't last too long outside compared to good rope, so if you decide to use something like that, I'd recommend having a couple of interlocking bowline loops in the rope or something that will "catch" the rope if the bungee breaks.
Generally speaking , I recommend things like rope *loops* through the tree so you can haul the antenna endpoint *down* after the antenna breaks and use of the right rope for the job (I like black Dacron).
"- I prefer setting up the antenna and leaving it there, if I can; Though I might be able to rig it such that it gets raised each time it is needed "
What you'll find with ham radio is that the time the antenna is needed most even for hobby reasons is that cold winter night when some interesting DXPedition has gone to a place you want to work, or the day you can't leave the house because of snow.
If you become fond of DX and strange propagation conditions like me, too, you'll just hate having to drag your antenna up. Every time I've ever had to set up an antenna before I could operate I would regret it, because I'd put it up and find nothing interesting DXwise and other times I'd have 10 minutes in between doing other stuff and knew some DX was on but didn't have time to put the antenna up *and* work it.
I like to do some ham radio in between other stuff often. Sure, I sit down for a long operating session, but I also don't read the newspaper with my cereal and coffee... I get on the radio.
Having to haul your antenna up really cuts into that kind of operating, so do it right and it'll survive foul weather with minor maintenance. I ran 6m (a very sporadic band) from an apartment with an antenna I had to set up every time because I wasn't "allowed to have it"http://n3ox.net/projects/sixmoxon/
But I decided to just risk leaving it up a lot of the time in the summer to try to catch the tiny window of opening to EU that would sometimes come in the morning!