Locuan made a post in a thread right below yours recently linking to this:
http://smashboards.com/threads/marth-data.379064/#post-18062624
Unfortunately, FSmash's range did get reduced and I dislike that too when it is one of your most high commital moves. But Marth's other moves have improved range from previous games. When you're comparing Marth's range to other characters' ranges in this game, I wouldn't use Marth's FSmash as a fair comparison (even though I agree that Marth's FSmash shouldn't be so short for its risk). Use Marth's other moves (Fair, Nair, DTilt, FTilt) instead for that purpose.
As far as visuals go, yes it's annoying for Marth & Lucina mains when their UTilt misses when it should hit. I think Vipermoon or someone else mentioned it could be a Z-axis issue. For Jab though, it might just be a visual desync because Marth players should know the hitbox location of Jab - it doesn't go all the way around you, but only covers your frontal area.
As far as improving goes, that takes time & a lot of practice. Have you tried labbing Marth just to take him out of a game context to visualize & know the hitboxes, startup & the end lag of every move he has? Have you thought about Marth's options in various scenarios? Have you looked into working on movement options for Marth? I can now consistently do dashtrotting/extended dash dancing (
https://www.youtube.com/watch?v=azRrqKUJ-5o), and it's an amazing tool in footsies if you master it.
Like every character, Marth also has some unique properties in some of his moves. His air Dancing Blades 1 has really low endlag, so sometimes I do short-hop to DB1 and treat it as a low-lag Fair. This can throw off your opponent's timing. Did you also know that air-DB1 also stalls you a bit in the air (most effective if you use it at the apex of your jump)? This can be used to slightly stall your recovery. Did you know Marth's Dolphin Slash (Up-B) has invulnerability on frames 1-5 when used in the air? And so on.
I didn't do much labbing when I started. I only got into labbing a bit because I dual main Peach, and labbing with her is fun. While labbing with Marth is not as fun, you do have the freedom and time (if you give yourself the time) to explore how you can mix up your movement, because maining Peach taught me the great importance of mixups.
Basically, what I've been trying to tell you that other than the fundamentals (knowing what your character does with each move and their strong/weak points in various scenarios), you have to be good at mixing stuff up in general. Maybe those videos you posted aren't representative of your gameplay, but from a macro-decision making perspective, you have to be able to be less predictable so your opponent can't choose the right option all the time to punish you. Mixing it up & taking a bit of time to lab your character is how I became a better player.