Chopper, you're missing the point again.
Remember the Laws are a framework that we use to manage the match. So:
So, if ref wasn't obstructing and throw was straight;
i) Should ref. play on or still blow with no trammer present?
IMHO, play on. Thrower has decided not to wait because perhaps he perceives a tactical advantage. Up to him that there's no trammer present, not due to intentional foul play on part of non-throwing in side. Play on.
Is this an eg of a premature throw (PT)?
As I understand your description, yes.
ii) No trammer, throw not straight opp. catches?
eg. of allowing adv. for PT?
The thrower has chosen not to wait for the trammer so IMHO has given up his right to expect a FK for it (unless non-throwing side were blatantly attempting to delay the throw by not putting a many in). He's thrown not straight and opposition have caught the ball. Advantage to non-throwing in side for not straight. And advantage very quickly over as opposition now have ball - which is the best result they could expect from a scrum.
iii) 1+2; throw direct to team mate.
eg.to allow quick back throw (QbT)?
As I understand it, you're describing a situation where a line out has not yet been formed. Assuming a Quick Throw hasn't been taken off the menu by the other ways it can be prevented (e.g. not same ball in use, etc.), then it's an allowable QT under the ELVs. Play on.
Watched Redruth v Stourbridge yesterday. Opp's throw on Red's 5m. Opp about to take a PT. Ref holds play pointing for Red to bring in a trammer when there were two or three players standing in GL in line with trams.
What could've justified this move?
Management.