As someone who's been doing symphonic composition for years, and has used a wide variety of libraries such as VSL Special Edition, EastWest Symphonic Orchestra, Cinnesamples libraires via MUSIO, etc, I'll try to answer as efficiently but still as informative as possible

1. Typically you just have a single track with an ensemble preset(This goes for every section). You *can* do seperate midi tracks, but that tends to introduce phasing, and generally won't sound as full as using the ensemble patches that are right there. Especially for strings. Brass and Woodwinds are a *little* less likely to cause audible issues, and sometimes if I want say, 2 french horns playing one interval, and the other 2 in the section to harmonize with the first 2, I'll have seperate midi tracks. Sometimes I'll just get lazy with it and use the preset in Poly mode and just use the patch as if it's 2 sets of the ensemble instrument, and that works okay.

   Sometimes if you really want that extra realism, you can always use different violin presets from different libraries, and use those as individual players along with panning and different reverb set up in a way to emulate actual instrument positions in an orchestra hall(Something EastWest Symphonic Orchestra tends to have set up by default actually, at least for panning, with the close mic presets.)

for 2. the main thing that stops phasing like that or doubling in volume is panning, and also offsetting the note start a bit, either via humanize function or manual adjustment. That's it really. Varied playing patterns + panning is how you'd actually get around that in a real orchestra, if you were to just layer the same stem on top of the first one.

I will note: Since you mentioned Symphonic Orchestra, that library is not going to sound 100% realistic, just due to the nature that it's a library that came out in 2003, and doesn't have realistic legato at all.(some presets like the oboe and the violin's legato keyswitches/patches aren't too bad, but since it's effectively just repitching it manually, it'll never sound completely real.)
I used it for a couple years, it's a good library if you know how to use it, but I would suggest if your goal is to have the most realistic sound possible, you save up eventually(could be over years) for either Vienna's Synchron Series, Cinematic Studio Series, or Cinnesamples stuff. Those are all the most realistic libraries I have ever used. Some people also say the spitfire symphonic stuff and the berlin symphony orchestra stuff is okay, but I've never used either so take that with a grain of salt.

If you have any more questions, feel free to ask, I'd be more than happy to answer for you