I think the way it works when you are in the hold pool is they tell you what the general requirements are for each airline (i.e. minimum average ATPL ground school %, number of retakes, maximum series/attempt for CPL/IR, maximum number of extra training hours required etc) and what the type rating situation for each airline is (i.e. paid for by the airline, paid for by the trainee, paid at short notice by the trainee) then you can work out which you are/aren't eligible for.
They then keep you informed of the selections that you are eligible for, but you don't have to accept a selection/assessment for that airline or at that time. e.g. if you wanted to work for Monarch and they offer an Easyjet selection you don't have to take the EJ assessment, or you might tell them you can only take a selection for airlines that will pay the type rating. Not taking an assessment opportunity that is offered doesn't count as one of your attempts, it only counts if you take the assessment and fail (or turn it down). But if you're turning down assessment opportunities they will be asking themselves why and probably be asking you too.
As for "required standard", from speaking to others, it appears that the non-technical skills training, each simulator session, and the MCC overall, is marked on a scale of 1 to 4...
3 being "good enough" or "achieves expected standard", 4 being "very good" or "exceeds expected standard".
2 is "below standard expected" and 1 is something along the lines of "oh dear me, try a different career"
Getting a 3 for everything is "good enough", getting 4s is a demonstration of your abilities, but won't put you at any particular advantage over anyone else, so if you get 3s, you don't have to worry about someone jumping ahead of you because they got 4s.
Getting a 2 for one sim is not the end of the world, but does limit your options with regards to certain airlines. Getting a 1 is a big problem.
Apparently airlines view a 3 as a normal standard, or "meets CTC standard". Since airlines have experience of, and are happy with, the CTC product, anything that meets that standard is pretty much a known quality and therefore has a good enough technical/non-technical foundation, with the extra capacity to be moulded by the airline and be able to cope with type rating and line training.
If you are "just another typical CTC trainee" that'll be usually be good enough for nearly every CTC-partner airline.
As for airline assessment/selection/interview, one of the main reasons people fail that stage is not because of their technical knowledge or non-technical skills (i.e. group exercises etc), it is because they don't appear to have thought about their longer term plans, or why they want to join THAT airline.
Granted, at that stage, most people would be happy with any airline that wants them, and even outside aviation, people at the start of their career are pretty grateful to get their first job whoever its for.
BUT, it's a game, you have to show them you want it. Know about the company, have a good answer for why you want to join that company, and why you are a good fit for the company. And if they ask where do you see yourself in 5 or 10 years time, don't say something unrealistic, but also don't say something like "flying long haul" at an Easyjet interview. Even though you've paid for your training and will probably be paying for your type rating, the airline is still going to be investing a lot into you, they don't want you to be leaving after a few years for something better. Even if that is your plan, even if they know it's probably your plan, for god's sake don't actually admit it openly, it's insulting to the airline who you're trying to get a job offer from.