pretty much got it in one Leon. Only thing I would clarify with your statement - and it may just be a language thing.
1. To vector a/c into the hold at any of the altitudes (flight levels) that is vacated in the hold at the moment
you have sort of covered that with this statement
Aircrafts always occupy flight levels in the hold from bottom to top.
But just to clarify, there may sometimes be 'gaps in the stack' (a controller may forget, or be too busy to keep laddering A/C down). It is conventional to put A/C into the first available level above the highest occupied level.
There are circumstances however when due to A/C priority etc, you can 'slip' an A/C into a level in the middle of the stack -
if it has been coordinated with all parties concerned.
There are circumstances in the London TMA whereby we regularly keep one flight level in a stack free so that we can fly departures through the middle - the stack controller will jump A/C down two flight levels to achieve this, when there is a gap in the departure traffic.
This is the convention we tend to use in the UK,
however, with coordination, we can do anything as long as it's legal and safe .