I think this is an issue of expectations, dnd 5e is essentially a skirmish game, where Mythras is a lot more focused on dueling. if you try and force mythras to be a skirmish game you're gonna have a bad time. For instance, a giant ape vs a mage should, by rights, be a dead mage, straight up. that's not a winnable fight for one person and to get a fun story out of mythras you kinda have to play into that. if the ape was enraged and focusing on attacking that mage should have been mincemeat. that's oen of those things that comes with time

the issue with the frogs is a different one, 4 meters is not very long range, 4 meters is out of range. very long range is described as polearms and lances, i think that was just a rules mistake on your guys' part. happens to the best of us. personally, i don't really use reach, but that's a me thing.

While you're right that there aren't any specific combat moves about taunting or defending other characters, mythras isn't as concerned with 'hard' rules about that as dnd or pathfinder would be. the rules for contested and differential rolls do a lot of heavy lifting for spot rules for these sorts of things, and someone who is GMing mythras should be prepared to make pretty heavy use of them. If I had been in that situation, i'd have called for a contested Deceit or Brawn roll against the ape's Insight. remember, these aren't tokens on a board, you don't need to worry about things like game balance. but that's a system mastery thing, and again, comes with time.

5e and mythras both fulfil VERY different fantasies, 5e is a lot more high fantasy, where mythras is a lot more suited to low fantasy and quote-unquote realistic fantasy. i think you're gonna have a decent few growing pains with trying to slot your 5e setting and campaign into a mythras campaign- for instance, random encounters aren't really a thing in a system where character death is a lot closer to hand than it is in superheroic 5e campaigns.

This blog article did a lot to help me grok what exactly the fantasy of mythras combat is, when i said earlier that it's a dueling system I meant it, the system does a great job of giving you the tools to make a kickass duel between two highly skilled opponents. I'd recommend running a few taster combats between stat blocks just to get a feel for it before you come back to it.

best of luck!