I mean the obvious solution to that first problem is that you shouldn't be throwing out unsafe options when you're standing at exactly the right range for lslice to hit you. The area that lslice threatens isn't actually that large, and if you're standing within the very small portion of that area where it can hit on the first active frame, literally just dashing forward will keep you from getting hit.
Also it seems weird to complain about this but not mana strike, considering mana strike can pose exactly that same threat, but Wizard can also move it across an entire range of distances and it's actually faster than lslice (try pitting them against each other in single player)
Yes, if you're standing at one extremely specific range in neutral, lslice is a strong threat. You can also just not stand in that extremely specific spot. And in that spot, none of his other moves are a threat.
If you're pointing DI away from the cowboy, the lslice loop should be broken after like the second or third hit. They can teleport after you, but if they try to keep using lslice they will very quickly get to the point where you can just DI up or away out of range. Also teleport uses an air option so cowboy can in fact not just keep teleporting and chaining mid-air forever.
Yes, lslice is a solid move that fills a niche in Cowboy's moveset. But to be honest you are making it out to be a way stronger option that it actually is. A good cowboy player is not going to be using it "every chance that they get" or even relying the threat of it in most circumstances. It comes out fast, but again it has a very specific range where it can hit someone even if they try to move and before they get to take a turn if they hold, and even then both the Wizard and Ninja have effective ways to avoid it safely.
Like Ninja for instance can just backsway at that range and unless the Cowboy just straight up hard reads the backsway specifically and commits to a move that will lose to most of Ninja's other options, they can't really punish it, while being hugely minus if they actually try to lslice. And of course Wizard can just mana strike, and that's assuming that their setups aren't protecting them to begin with (same case for Ninja actually).
lslice is just simply not an even remotely good enough move to carry a bad player unless their opponent is also a bad player that isn't even considering that the move exists. Lslice looping is also frankly just bad if the opponent DIs correctly, it quickly builds up a lot of knockback and does pretty sub-optimal damage.