Any non-moving unit (and only issuing a move order has you moving) can receive a support hold. But note, even attempting to move negates the support hold, even if you bounce. So support hold can support a convoying fleet, a support holding unit, a support moving unit, or a holding unit, but never a unit attempting to move.

Yes, but if it's a unit that you're trying to support-hold, then it's something you presumably are anticipating an attack on. Which means that that support will likely be cut. But it never hurts to try.

Not completely true. You may be supporting an attack on a unit you expect to attack you. Seeing as a unit cannot cut support against itself unless it dislodges said unit, supporting the supporter could make the difference between the supporting unit being dislodged or the enemy unit being dislodged.

A hits D with B support
C support holds B
D hits B with E support

A moves to D and B holds it's place

But if C doesn't support B, A moves to D and D moves to B.