Marco's ability is to regenerate from wounds, but with a limit considering the explanation given by oda in the SBS, so why wouldn't a strong enough punch send him flying? Marco is 100% solid, anything with enough strength can do that.
Haki won't bypass the healing marco is inherently capable of, its not the same as with being rubber or a logia.
Not really.
Garp's fist cannot be stronger than the force of light.
Garp's punch had to be have been packed with Haki when he hit Marco. When I say Haki I mean Armor Haki btw.
I'm almost 100% certain no human in One Piece has a non-Haki punch stronger than the Force of Light.
Marco ultimately took all the damage that punch was supposed to give, he merely would have healed right afterwards. In general terms I would argue that armor haki has the ability to make contact with things which would normally be hard to make contact with which is why it is effective against logia and people with weird bodies. Allowing contact in itself won't really allow you to bypass an ability which does not present an issue of contact (which gets us back to marco's 100% solid body).
Its possible for you to use your Armor Haki in an offensive manner though.
People have different levels of strength with their Armor Haki too no?
The Werewolf idea is the same concept as Marco.
Its almost impossible to damage,
Think about the One Piece series,
Marco took beams of Light and a Kick from Kizaru comfortably, that same kick sent Scratchmen Apoo through several stories of a building, sent Urouge through some buildings and pretty much wrecked Drake and Hawkins with some kicks.
Marco's body comfortably stopped that kick.
Garp came with beast level Haki and punched Marco to the floor.
Surely its that Garp is able to create harder punches (through Haki?)and the fact he caught Marco way off-guard that allows the rule of Marco not taking damage to be bypassed.
As I said, nothing in One Piece can bypass Haki so far.
(Literally the Yami Yami no Mi is the only thing which seems to be working its way around rules but then we can't count that since its part of the story's main Plot Twist)