How long before your first meets should off season training begin, I mean like practice meets that are about 6-8 weeks away from the major track meet of the 1st phase of the season?
I think 6-8 weeks will suffice if that's what you've got. For high school athletes playing other sports I'd try to use that as GPP.
To keep motivation and interest during offseason training what tests are there that can be done (100-800,lj,tj athlete)?
I test 30m, 150m, 300m, SLJ, STJ, BLF, OHB, Squat, Clean….not necessarily all during one test cycle.
Can short sprint tests be done or should all tests be longer, eg, 300 or 600?
It depends what your objective of testing is. For example, if you are trying to assess the training adaptation of speed endurance you'll want to test something longer than a 30m.
Can you run on an outdoor track during winter? (flats i'd presume)
Why not? It depends how cold it is…if it were under 50 degrees I probably wouldn't try to do all-out sprinting outside.