"Should I start strides a couple times a week or should I just do two weeks of easy running to get back into it and then start incorporating strides?"
I don't know what you mean by strides... 100m sprints? 400m intervals? You can fine-tune your speed during the xc season. Summer is all about getting base miles in.
"Should a tempo once a week also be done after the first two weeks, or should that be held off until after a month or more of base already?"
If I were you I would start doing tempo runs your first week. I have had tremendous success doing two tempo runs a week. A short one the day after my long run, and a longer tempo run two days after the short.
"I plan to start at 40 miles a week and build up to 75 (with doubles). If I am doing 4-5 doubles per week in late August, will it be bad if I stop them once September and the season starts? If I want to stop doubling would anything bad happen?"
If you drop your weekly mileage when season starts, you will eventually lose that extra "base". In high school, I never kept my mileage much above 50 during the season, and by October I noticed my third mile time was not as fast.
"If my goal mileage is 75, should that be hit at the very end of my base or during the middle? In season I'm assuming due to the rest of my team that we won't go past 55 miles a week. So will a 20 mile drop be bad for me or its fine to drop down significantly when base phases out?"
Get your mileage up as much as you can as fast as you can. I imagine a good progression for you, depending on how much you did during track and how long you've taken off, would be
the following miles per week (mpw) - 30, 40, 50, 50, 50, 50, 60 (still all single runs), 70 (start 2-a-days), 80, 80, 80, 80.
During the season, you can keep your mileage up (at 70 mpw or more) by doing AM runs before school, and then still practice with your team after school.