Do I detect a slight edge to your last post Connie? It comes to something when you can't even go away for a few days without being missed on pprune!
To answer your questions, competence can only be managed by a unit. Continuous monitoring and annual checks will show up anyone who is having difficulties for whatever reason. Of course, work colleagues will usually pick up the signs even earlier. The unit management have a responsibility to ensure that people doing jobs are competent to do so - for many, many, reasons aside from licensing. So, if a unit manager becomes aware of someone who is not competent they should do something about it - the problem is what they should do. There are often few options, controllers and assistants are employed to do those jobs and there sometimes no other jobs on the unit that require this skillset (and we already know they're not up to scratch anyway).
You are looking for a licence for the wrong reasons. The CAA may issue licences and so the CAA can take them away - but this should be a last resort or the last stage of preventing someone who is not competent from endangering others. The first step should be at local management level - what you seem to want is for the CAA to be the bogeyman that takes away the licence while unit management wring their hands and say it's not their fault. You don't need a licence because, in simple words, if someone can't do their job they should be sacked. It sounds harsh but taking away a licence they need to do a job has the same effect in practice.
One would hope that lots of avenues are tried before someone in this position loses their job but the bottom line - and I think we agree on this - is that someone who is not competent must not continue to work in ATC.
As you say, payment for extras like training are secondary to ensuring competence and can be dealt with separately - and don't need a licence either.
I said that some units are better than others - and that applies to competence and usually all other things. A good manager (be that a Unit General Manager or Watch Supervisor) will be aware of how their patch is running. They'll spot problems as soon as they arise or, even better, will have an open door for someone who's got problems to go and talk and, hopefully, sort it out long before it becomes serious or can affect other. A unit or watch with a good manager will have some system that ensures competence that does not require a licence. A cr@p manager will actively deter such openess and when someone has a problem the first thing that will officially be recorded is an incident that gets reported to the CAA. Then the 'difficult' decisions are taken by the CAA.
OK, these are two extremes but I hope you get my point.
Finally, your point that controllers do not understand the ATSA's job is probably valid in many cases - hence my claim that they should be able to do the ATSA's job if necessary. But this again is a unit thing - a controller who may be called upon to do work normally done by an ATSA should be trained and considered competent to do it before being signed off as competent to work in any particular position.