My concern regarding FLR/ILR is simply my fear of not passing the LitUK test in time. My first attempt will be on the 14th. I would have time for only one additional attempt if I fail the first one. That's the only reason a third FLR(M) is being considered.
Ah, okay - I thought you meant that you wouldn't have enough time between the 14th and the 26th to attend your appointment ... which is not relevant anyway, as it's the online application date that matters, not the appointment date.
Having said that, as long as you have studied the Life in the UK test book and taken practice tests, you should be fine. It's not often we hear of someone failing and having to take it again.
But if that was the case and you couldn't retake it before the 26th, then you would need to apply for FLR(M) instead of ILR... but I wouldn't plan for that just yet.
Right now, just concentrate on taking the test. There's no need to add on extra stress worrying over 'what ifs' at this point. If it turns out you need to apply for FLR(M) instead, then you can deal with it when it happens.
If it helps any, we had someone apply for ILR online before they realised they had to pass their Life in the UK test first (they thought the test was part of the application process). They managed to book and pass the test in the time between submitting their online application and mailing their documents.