In general a quite balanced unit imo. Thing is, the menavlion is not that fast or have any good handling. If you are up against this weapon, know it's limits, block the first strike and move up close and don't let him get away. Play aware and you will beat this Menavlion 9/10 times.
If you are playing with Menavlion, don't let the enemy get too close, play aware and you can beat your opponent as well. The menavlion can deal tremendous damage but you can't let enemies close in on you.
Sometimes I feel however the menavlion seems to be able to hit very close to yourself so to speak, but then again, it's not the only weapon that sometimes seems to have weird hits register as good hits.
The fact that this unit has no shield is interesting. Might make it more balanced as you have to scavange a shield if you play this class.
While you are correct in your statements, most experienced players will be able to beat mostly anyone of an inferior skill level, in groupfights they can be an issue. They can also be an issue against new players mainly. New players should not necessarily be punished so harshly in their first fifty hours by a strong class who can outreach and spam.
Experienced players can beat any newcomer with any character, regardless what the inferior player is playing. If anyone is inferior, they will lose, that's the definition. Menavlion is not the only thing that can one-hit-killl you in this games, a lot of the one-handed weapons can, you can get headshotted, a cavalry can take you out. All of these, of course, might not be super fun to experience as a new comer - dying so fast, but they are not the whole game and only a part of it. Everyone will die to backstabbing cavs, headshots, and so forth, until they learn to play and recognize threats from a distance.
You are right, there are other threats that are as big as the the menavlion, but headshots are not really one-hit anymore. Cavalry will always be powerful when backstabbing and there's little to do about it in a 2v1 situation or if the player is unaware, and that is where learning the game comes in. I've not yet seen one handers perform one-hit kills however.
Menavlions do not belong in multiplayer, they will never be balanced as the length and damage will always be too much, used in a group fight they are unbeatable, just sit behind your shields and overhead everybody. I think removing it altogether and replacing it with a different twohander is the go.