It's been a long time since my last Starcraft games, but if I remember correctly...
1. Do resources deplete over time or only if forced mined?
Only if force mined. You can do it two times with each resource. Some event cards also deplete them to half, or completely if they're already at half.
2. How does the resource gathering work simply put?
Not sure if I understand the question, but to get resources, you only have to move one of your available workers into the resource card, which you have by having a base on that planet/zone (additional cases can happen, like different armies bases on same planet, see rulebook).
3. Simplify the regrouping phase?
Well, on the rear of the rulebook, you've got a nice step by step sheet. There is a simplified regrouping phase point by point, all 10 of them.
Not sure if you want a concrete point answered.
4. Transports are only placed in navigation spots?
Yup, they serve as 'connection' points through planets. Always in navigation routes.