This is how I see it.
1. We do not know what Boba poisoned Jodo with, they only effect we observed was instant paralysis. So we do not know if the poison itself was lethal.
2. Boba does not rely upon the poison to kill Jodo, he also leaves the damaged jetpack alluding to the possibility that the poison is not fatal.
3. We do not know what happens when the jetpack detonates, Jodo may have managed to get the antidote, and just not made it clear of the jetpack explosion.
4. The final frame shows Jodo's arm poking out of the rubble, and while clearly some rubble fell on him, it does not appear to have been a great deal. The final battle sequence takes place in a large room with a cathedral-like ceiling, Jodo is left there poisoned with his soon-to-explode jetpack. But, if that jetpack explosion was sufficient to bring down part of the structure, that would have completely buried Jodo. The fact that a part of him is exposed leads me to think that he managed to get to the perimeter of the detonation and only had small amount of debris fall on him.
Finally, I never thought of Jodo as stealing Boba's identity. In his character sheet it mentions how he patterned himself after Boba, NOT how he pretended to be Boba. Secondly, in Twin Engines of Destruction, it is at first Dengar who merely assumes that Jodo is calling himself Boba to get the higher bounties; and later Boba is informed of this from Dengar and decides to deal with Jodo. And when Boba sets up his ambush, he contracts for Jodo through the House Benelex Bounty Hunter's guild. He doesn't ask for Boba Fett and get Jodo Kast, he asks for Kast and gets him. I got the impression that it was merely the uninformed layman that mandatorily assumed that any Mandalorian armor-wearing bounty hunter was Fett that instigated the misunderstanding between Fett and Kast of who-did-what-to-who.
All in all, I think Kast was a pretty cool younger version of Fett. And I don't think it is all that unreasonable to think he survived, starnger things have happened in Star Wars. Neither do I think he was some cheap hack, he killed the previous owner of his mandalorian armor, Bobb Feskitt, who was a Mandallion Giant raised as a Mandalorian and was definitely not a pushover.
"Home is where the armor is."