Answers

If it is sawdust it is bugs. Don’t usually see them in treated wood.

It is a must! Be sure that it is dry before staining.

You should have a structural engineer look at it.

Needs to be sanded off and silicone should never be used on wood. Use only a good latex caulk made for log homes like Energy Seal.

Call a local exteminator

Back to bare wood.

Apply Shell Guard borate per the label. Call 800-564-2987 to buy.

Sounds like it may be powder post beetles. The only good way to get ride of them is to strip the house and apply a borate solution.

With solid color you can use any color of the rainbow.

Could be. Hard to say without seeing.