Obviously it's not the jab itself that kills, but it confirms into back air on the entire cast at basically any % you're caught on. It's also super safe to throw out and covers many ledge options, what with its several active frames. Roy also has this, of course, but just as with anything regarding the two characters, it requires far less skill with Chrom - heck, Chrom's extended range means even jab to forward smash can be a kill confirm a lot of the time. And yeah, that kills ridiculously early.
I've been killed by Chrom's up B while on stage more times than I can count. If he's using it near the ledge, the best you can do is to just sit back and accept it, unless you have a good projectile or ridiculous disjoint like Byleth or the Belmonts. It is possible to edgeguard it, but getting killed at 0 is too big a risk. It's the same with Ike's, but Ike's Aether at least has a hitbox that makes some sense.
Also, Ike's Aether isn't a good out of shield option, unlike Chrom's, which is why the 26% is a big deal. Everytime you hit Chrom's shield, you eat 26% for free.
Still no. I'm not going to spend any more time trying to find out when exactly jab to bair kills, but it's definitely not at 70 except maybe on Jigglypuff of something. I'm also fairly sure it's not a true combo. Jab sends way too far to combo into FSmash at KO percents. Maybe you could nair into it, but that'd have to only work at a pretty limited range of percentiles.
Chroms jab is fairly slow for a jab as it comes out on frame 5. It has like 3 active frames. And it has 10 frames of lag, so it's not incredibly safe.
I honestly have no idea why you're getting killed with Chroms Up B more times than you can count. Did you mean off stage? There's close to no way he can kill you with it while off stage without also killing himself. And if you are getting killed, you probably went way lower than you should have. If you hover at edge level behind him, his only option is to try and B reverse it, which would kill him too.
There are really tons of vulnerabilities here. Charizard or Bowser can make sure he eats a big chunk of fire damage before he gets back. Palutena can throw out explosive flame which will most likely hit him and has o risk. Any character with a counter can wreck his recovery if he's coming from beneath. Pikachu and Pichu can harass him with thunder jolt, Ivysaur can hit him with Dair without going low enough to put himself at risk, or bair, or fair with very little risk. Mii Brawler can hit him below the stage if he has the shot-put, Villager can safely f/bair if he goes horizontal, and can Fsmash him if he's coming from below, Squirtle and Mario can Fludd/Watergun him if he's far away and he has very limited, Piranah can force him to take poison damage or hit him with a spike ball, etc etc.
Most characters can at least score free damage with next to no risk to themselves. And even if you do have to take a risk, you have a good chance of succeeding, and if you don't he'll almost definitely have to kill you to stop it. You may not want to try at 0%, but most of the time, you're not at 0%. His recovery is easily among the worst in the game.
As for the Up B as an OOS option, it's good, but you're ridiculously overselling it. It comes out on frame 10, which is much slower than a grab for most characters, and much slower than most OOS options. Even if you're frame perfect, a lot of aerials will be entirely safe on it unless you parry, and even then some will still be. If it misses, you're open for 30 frames. And 26% is nice, but at most percents, alot of characters can get that off a safer option like a grab. For instance Charizard dthrow to fair which does 24, and if you get a pummel in, you're basically even with Chrom. Granted it has a bit less range, but it is faster with significantly less risk.
So... he's got a decent kill confirm, and a nice OOS option... Can't see that destroying competitive balance. You gotta go easier with that ban hammer.