By the time the "product" gets to the FL it will have passed through the hands of both TPs and Evaluator Pilots (EPs). These EPs are the "traditional" OEU function, and provide the Operator input to the capability to ensure, as much as possible, that the FL gets the kit it needs - though sometimes not the kit it wants. On 41R these roles are fused with TPs, EPs and Trial Management Officers all on the same unit, and I believe it is the same for 206R. It's slightly different with RW; the tri-service RWTS is TP only, whilst the JHC OEU covers off RAF/AAC needs. The RN, interestingly, don't have a permanent OEU preferring to stand up, and then stand down, "W" suffixed Sqns for entry to service.
Re pre-cse quals, it seems the system is happier now to allow QFI/QHI and TI/WI to attend ETPS when it helps - eg CtoI on core types post course and to help the EPs with Course Development etc as part of a CTT.
As for post TP careers, well there remain a few posts at BDN for promoted TPs to backfill (CTP, DCTP, Wings etc) but most need to step away from the TP world to advance - Manning will counsel you prior to ETPS that you will be prejudicing your career by attending (time away from the FL, not a 'crunchy' Staff Job, tall poppy syndrome....). Jobs outside are not as plentiful as before, so more TPs will return to the FL in future; as usual the ebb and flow of Industry and programmes will dictate what happens post a TP tour. In the RW world many TPs simply join the pool of civ RW pilots in Offshore, PAS and HEMS when they leave.