Yes, you can you will need a very light skim about 2 times the size of the patch to help feather it out get a nice metal drywall knife and this will simplify the job
Yes. With larger holes it may take a couple applications because fresh spackle will retreat a little in the hole when drying. Just be patient and allow drying.
Use bondo patch kit $6-12
I bought some of this for a project a year ago and then just used it again last month. It will keep at least a year assuming it is sealed up.
It is premixed. No water.
Yes but one made for wood will allow you to stain it with best lasting effects
This is in-between what you are describing. It is a thick, pasty material. Made especially for nail holes. Dries fast and paints over well.
In the past I have used screws, chopsticks ends, and dowel pieces to partially fill the hole and then preceded to fill and cover with the DAP.
No. It doesn't work anywhere on anything.
For this formulation applying a thick coat is not a good idea.