In a world where there's the system The One Ring, I don't see a point in doing any other Middle Earth system. Perfection has already been achieved (for this specific setting).
Concerning high-level play, I think having way lower HP for everybody would also fix a lot of things, since the main issue is that battles take forever due to having to whittle down ridiculous HP sponges.
In this case I'd actually not mind that much, since D&D5e and Diablo both are pretty much the same genre (High Fantasy with a lot of fireballs). Maybe slice all HP in half to make it a bit more lethal on both sides, but that's it.
It's way worse for stuff like "Adventures in Middle-Earth" and "Beneath the Monolith". Like, how? Those genres aren't remotely the same.
I think you just posted a link to your thumbnail image, not the actual article.
"Beneath the Monolith" was also put out by the same company that produced the original setting with its bespoke system (Numenera/Cypher System). They just know which way the wind blows and strive to maximize their profits.
Multiclassing is an optional rule in D&D5e, not allowing it should not be controversial.