If we agree that A50 may or may not be revoked by virtue of the absence of any mention in the rather succinct, plain text, then surely by the very same tokens, conditions may or may not be attached either way.
No. The treaties cannot have any additional conditions attached. That would be a change to the treaty. The treaty, as written, is either revocable or not. If it is revocable, then we can revoke. If it isn't, then we can't. Simple as that.
UK plc could just as easily say that it wishes to rekove A50 but only on condition of halving our contributions*
You can ask for anything you like, but unless the treaty allows it, it can't form a condition on invoking a right granted under a treaty. It could happen if all EU28 and the commission and the EU parliament agree to it.
in the same way as the EU could turn round and say that it isn't revocable and as such we must leave and rejoin as per A49, and then only with xyz terms
Yes they could claim Art50 isn't revocable, and then the ECJ decide if it is or not. If it is we can revoke. If not then A49 is the only way back in.