No, that's not the ruling about Sapling vs
Morgul Brute. That case is about "preventing an action that would prevent another action." That's a logical double negation, so the original effect is not prevented (is being "prevented from being prevented").
If the FP player chooses to add a burden, that's
Morgul Brute's original effect and not a prevention cost. Then
Melilot Brandybuck can freely prevent that burden, and that's all.
The other ruling you are referring ("an action needs to be played out as far as possible") applies to
Deathless Lord. If you cannot wound a specific ally 2 full times, you cannot choose that ally and must either exert a companion or wound another ally with 2+ vitality. If all companions are exhausted, you must wound an ally twice (if possible). If all companions AND allies are exhausted, only then you can choose to wound an exhausted ally.
Another case is
Boromir BoC (carrying The One). At the start of his skirmish, you must wound him twice or add 3 burdens. If he bears an
Armor, you cannot fullfil the 2 wounds, so you're forced to choose the burdens. But if he has 2+ vitality with no
Armor, you can choose wounds (and use Sapling /
Intimidate /
Arwen Elven Rider to reduce them to 1 or 0), or add 3 burdens.
It's a fork in the road: you choose A or B (or C... if there's more), but after choosing one the other doesn't matter at all, it simply vanishes in the distance.
Morgul Brute is a single road, with 3 stages: 1) you can spot a Nazgul, 2) if so, add a burden, and 3) the FP player can wound the RB to prevent that burden. No simultaneos options.
This doesn't answer to your call, but hope it helps anyway.