Keeping McKenna is gonna be key for him imo. Fletcher's gonna replace Solskjaer as the head coach and will obviously face the media. And I think we sill now hopefully see McKenna coach the team in a more expansive
There's 5 levels and it seems Fletcher was completing level 3, last year at the club, which requires one to coach a junior team which he did with the u16s. He should be at level 4 + at the least and that's good enough for him to become the head coach in the professional game from what I've read online.
B License is the U16 one, that's also the third level of the UK coaching system you refer to, it's the first UEFA level, UEFA A License is the U18/reserves one. (UK Level 4 / UEFA 2)
You need a UEFA Pro License to manage a top-tier league on a permanent basis in Europe, also you need it to manage in the Champions League.
Interim basis is allowed, and iirc you have a grace period if you're working on attaining the license, but I seem to recall there was a manager that had to sit out a Champions League match a few years ago due to not having attained the license yet.
I also think Fletcher should be on Level 4 / UEFA B this season, as you say. So not sure he'd be allowed to take the reins for the Young Boys game.