1. Should I install my backflow prevention right at the tee in the main line or back at my sprinkler manifold?
It makes no difference as long it is before the sprinkler valve manifold.
2. Should I sleeve the PVC that is under the path to my side yard parking? If so with what?
No real reason to do so.
3. I plan on using 1Ē Wirsbo pex for the main from the meter to the house, commentsÖ
Whatever makes you happy.
4. What should I sleeve the main with that passes through the footing into the crawl space?
5. Should I move the irrigation supply line farther away from the trees (Iíd rather not trench through the middle of my yard but am I asking for trouble with the roots)? Or option 2 is to tee in the crawl space and have no trenching to the sprinkler manifold.
Put it wherever you want to.
6. If I do a pex home run system should I install the manifolds in the garage by the water heater or in the crawl space?
You put the manifolds as close to the final connections as possible.
7. Would a branched system make more sense?
There is no right or wrong. It is whichever you prefer.
8. My electric service panel is grounded through the galvanized water pipe. After I disconnect it from the meter can I leave it in place so I donít have to mess with re-doing the ground?
As long as it remains connected to the old pipe in the ground. But a dedicated ground rod, or two, might be a better option.