I appreciate your comment! Your theory of what difficulties are feasible with Only Marth is very interesting. I assumed from the first chapter, that playing any further than Hard-1 could cause a soft lock well into the game. My general rule of thumb with these challenges (even including non Fire Emblem games) is that if the first level cannot be beaten then that means either the challenge is too outlandish or the video is going to start off weakly. I feel that not as many people are going to be interested if I start off by saying "Well, the first level in and I can't do it". I thought that the difficulties harder than Hard-1 at the beginning were too much. I've been thinking about revisiting this just to see if Marth can make it through on Hard-2 (or possibly Hard-3), but not as a full video like this one as I may end up repeating a bunch of information from this video.
Also, I've noticed after making that video, that someone on GameFAQs did do something like this back during the DS's hay day, they said they did it on normal. However, I cannot extrapolate anything other than Marth's final stats. The non-capped stats are higher than what I achieved, which means he got really lucky or, more likely, managed to get the Secret Shop items (which were far out of my grasp and I don't know how easy it was for them to get them - that post is also >10 years old).
I am convinced that after doing this challenge, the value that the RNG table returns is based on how many times a unit has attacked, has been attacked, evades an attack or misses an attack. When I repeated strategies and loaded save states, so long as the units are in the same position as before, there is a very good chance the same outcome will happen - the true randomness comes from how the enemy and player unit(s) moved and are placed. To keep it brief, the RNG is locked way before anyone fights so there is not much point in save stating and repeating the same actions - as the same outcome will happen.
You mentioned New Mystery of the Emblem as well. I researched if anyone has done it with Marth only, but so far it seems to be quiet (possibly because it's exclusive in Japan). I've got my sights set on that at the moment before someone else does it. And that means that I will not be playing the PAL version.