It's pretty easy to edgeguard Marth. Just hang on the ledge, and when Marth is about to use his up-B, drop, grapple, reel in. It works almost every time. If Marth make it back over you, you can easily use a ledgehop fair or uair for a free 12-15%. To a lesser extent, dropping bombs can also edge guard Marth, but only on Final Destination, because Marth will most likely get caught under the lip of the stage. Missiles and Charge Shot are very ineffective against Marth because of his fall speed, disjointed hitbox, and fast up-B. Zair can be used right as Marth double jumps to hinder his recovery further. If it's airdodged, it gives you some time to prepare an edgeguard.
You don't ever want to be over a full hop height above the ground. Marth can easily take advantage of Samus' falling speed with his own. This is because if you airdodge one of his aerials, Marth will still be in position to hit you with a different aerial. It is extremely difficult to remedy this problem. There are two main methods I use, one of which you already use. Drop a bomb, DI away from it, then move towards it. The bomb discourages Marth from getting too close, while DIing around it baits Marth. Next, there's drop a bomb, DI away, drop another bomb, DI either direction. You can use this to get off the stage where Marth can't double aerial you as easily, or to get in between two bombs to limit approaches.
If you find yourself off the stage purposefully or recovering, there are some strategies you can try to avoid getting edgeguarded. First off, save your double jump and only use bombs outside of Marth's edgeguarding range. This range differs from player to player, depending on how aggressive they are. Don't be afraid to retreat with bombs in the air. You can often use this in conjunction with a bomb jump so you don't lose too much height. When you find the time is safe, double jump+airdodge. Samus has a very slow falling speed, so the airdodge should finish by the time you reach the apex of your jump. Then up-B/grapple. The airdodge is used to prevent any gay Marth stage spikes. Using projectiles while offstage is discouraged because you lose horizontal momentum when using them, which hinders Samus' good recovery.
Remember, Samus can ALWAYS recover, it's the player that makes the mistakes.
Your spacing game is very solid, about equal to that of Honu's. Both players demonstrated turning the tide of the battle with good spacing. I'd say your spacing skills are much better than mine, so I can't really say anything.
Your ground game is rather good. I like your use of jabs and retreating. However, I don't like your use of grabs. I remember a moment in one of the vids where you used a SH zair, which lands. You try following that up with another SH zair, which gets shielded and punished. This is an example of predictability. When Marth got hit by that first zair, it was like a reminder that said, "okay, watch out for Samus' zair". Instead, you should SHFF-dash grab. The SH makes Marth think you're going to prepare another zair and the grab has relatively the same animation as zair which may get you a free grab.
Whenever possible, try to throw Marth onto a platform, because it sets up for a very easy tech chase. When platforms are inaccessible, I find fthrow to be the best throw. Dthrow puts Marth too high in the air and makes your follow up too easy to read. Fthrow puts your opponent in the air but closer to the ground. You can follow it up with a zair, fair, dash attack, running usmash, or dashing shield. There are plenty of other options, but the ones I listed seem most effective to me. A zair keeps you out of Marth's range but is easily airdodged. However, landing from an airdodge doesn't necessarily mean Marth is safe. Marth gets about 4 frames of landing lag when landing from a throw, so you have some options like predicting Marth's next move (shield, dodge, attack)... because we all know 4 frames isn't that much
. Fair has a long lasting hitbox and puts you close enough to Marth to follow up his airdodge, however, his fair beats your fair. Dash attack causes about the same effects as fair, except you're not jumping when you use this which may influence your opponent's decision. Oh and dash attack combos into Screw Attack. Running usmash causes relatively the same effects as the dash attack, except it instantly punishes his airdodge upon landing. Dashing shield is your own technique, so I'm sure you already know more uses than me.
Marth's airgame>Samus. No, not Samus' airgame, Samus in general. ...Except when Marth is above Samus, because Marth's dair isn't a good approach option. Sorry bud.
Hope I helped...
