Answers

You shouldn’t leave them bare any longer than you can help. Mold spores and pollen will settle on them if left too long. You need to media blast and sand or Osborne brush after blasting.

I would recommend waiting at least 6 months from time of being cut down. Debark ASAP.

I would let dry for a couple of weeks. You should also give it a light sanding with some 50 grit to remove the mill glaze to avoid peeling later.

Nothing that is good and cheap. You will always have some maintenance every 3 to 4 years. You would be better to strip it all off and apply a good borate and let go natural gray.

I would go with the cement board for siding or Hardi Plank.

You should be able to put house wrap (Tyvek) on first then just screw the logs on. Check your local building codes.

Any rotted logs need to be replaced first. Then, ideally, the logs should be stripped and treated with Shell Guard borate. The rest of your plan sounds right to me, but you should check your local building codes.

Leave them as they are. If you start trying ti fill them it will only cause more problems.

You should use The Sikkens marine line of products.

I would dig all of the rot out, apply Shell Guard borate to area then fill with Wood Epox. Call Dave at 800-564-2987 for the materials.