Delver Sergeant question

A Delver Sergeant has an allied champion. The Delver Sergeant has not used its champion power. The Delver Sergeant's allied champoin is destroyed.

Next round, the Delver Sergeant is the only champion left on the team, it hasn't used its champion power yet. Can it still use one of the powers that its (now dead) allied champion had? 
No, since that piece is no longer in play. I assume this would also be true of a champion piece with some type of Delayed Appearance, though that is a  bit less clear.

Yes - the piece would seem to have to be in play.


 

So, in order for a piece to be considered an ally, what are all the preconditions?

1) It must be in play.
2) It cannot have been destroyed (perhaps this is covered by #1).
3) It must be on the battle map?
   3a) It must have joined the fray, as it were, if it started off the map.
   3b) It cannot have been removed from the map (i.e. Sarcophogas, etc.).
   3c) If it is a piece which can leave the map and then return, it must not be off-map
4) It must be active?

By the way, issues such as this have led me to consider working on a DDM glossary. The Battle Rules document is nice, but, if you're not sure where to look for something, you might have to do a lot of searching before you find what you're looking for. (An index would be useful.)

But, a glossay would be cool too, where things like this "Ally" are defined, and then there are relevant links (whether hyperlink or otherwise) to the supporting material in the Battle Rules.
1 actually covers 2 & 3a). 3b) and 3c) are not clearly described in the rules, and are actually a kind of a problematic issue. 4 (being active) is not required to  be considered an ally (kind of childish, but I was missing a space here and it made the sentence look totally different).

Please post your Glossary when you finish it, we would love to have it!
A piece that leaves the board but still takes turns is in play.
A piece that has not yet entered the board, and does not take turns, is not yet in play.
A destroyed piece has left play.
An inactive piece is in play.

How do we know which pieces that are not on the board are supposed to be taking turns?
Victims of Sarcophagi still take turns, obviosly, as do Chain Golems' victims.

Not sure, but it seems that Will'o'Wisps or Spectral Panthers would not take turns while they are on sabbattical.
(But these are not relevant for the Delver Sgt question)

Funny (and somewhat relevant), is that as the wording stands now, CP of a champion that temporarily belongs to Your warband - is an ally - while performing an action while controlled by You, might be copied by Delver Sgt (or Wulfgar)