Original works are automatically copyright of the artist under the Berne convention. You do not need a copyright symbol. If you visit the legal corner there are many threads on copyright and this has been discussed recently here.

2. Yes you can legally, but IMO you should inform the buyer thast you may/will use the image for reproductions.

3. No, not legally, but as in 2, morally I would inform purchasers of the original.

Art by Greg Long . My Blog

Copyright is automatic and you don’t have to state it anywhere unless you want to. Most governments have a place where you can register a copyright, but it will cost you money and is unnecessary.
When you sell the original you are not selling the copyright. It belongs to you for the rest of your life plus 70yrs or so, so your children will inherit the copyright. You can do anything you like with your own image, reproduce it as many times as you like, anywhere you like.

Yes you can.

You should make sure your buyers know that they do not own rights to reproduce your work when they make the purchase. The rights remain your own but buyers may not know that so you can guard against their ignorance with some sort of written communication. You can sell work and include that permission (for the buyer to reproduce) but it is not the norm. Even if you sold them the rights to reproduce you would still have the right to do so yourself unless you sold them exclusive rights to reproduce meaning they would be the only one allowed to so, anywhere. This tends to be applicable with authors. they can’t sell the rights to publish their work to a big company and then also self-publish the same novel and sell it themselves as well.

I think placing a copyright symbol on your work would appear amateur in most circumstances especially anywhere on the actual piece, although if on documentation it might be okay. But remember that you are only informing them, your rights already exist.

This is true, but it would still fall on the artist to hire a lawyer to defend their copyright. Many artist don’t have 1000’s of dollars laying around to do that so for some if they own the copyright or not may end up being a moot point.

The question is asking whether the painter can reproduce their own work (yes) not whether the buyer can reproduce the work (no). That is a whole other topic.

Btw, while we’re going off topic. Solvent does not make paint leaner, it makes it thinner. To make paint leaner you would have to, either, remove oil or add dry pigment.