My post further proves the difference to be no where near as extreme and capitalises on myrrth being much more versatile.

Your post compares wildly unrealistic situations for every unit involved. Your matchups are almost never going to be merge for merge when pitting Myrrh against horses and need to be compared as such. Same thing for the Myrrh/Nowi arena comparisons. Myrrh's merges are almost always, if not always, going to be lower on average, so her effectiveness is greatly reduced.

You can't compare a merged unit to an unmerged unit and claim it to be a fair comparison.

I absolutely can, should, and will. +10 Myrrh is unrealistic for most people. Unmerged or low merges is going to be the norm. But for a unit like Nowi or Fae, merges are very accessible and you should be comparing as such. Enemies in PvP modes are almost always going to have better skills and more merges than yours, unless you're at the very top of everything already. When you're working with 3/4* units, though, the max merges are accessible.

She clearly is able to take on both reinhardt and lyn in their best forms and as fire emblem heroes is concerned, winning and losing is all that counts. Overkilling or remaining HP is not calculated in your final score.

If they both hit her on the same turn, she's entirely out of the fight, even in your optimal (and unrealistic) conditions. If Rein has buffs and +atk, she's out of the fight after combat with him alone. That's not the makings of a good counter.