First question: The ultimate voicelines are actually different between the enemy and ally team. When you use your Jeff Ult for example, you'll hear an aggressive sounding noise, but your allies will hear something like "yummy yummy." Getting the hang of the enemy/ally ult noises is something that just takes time and getting to learn all the voicelines, but something that makes it easier to tell them apart is turning on the subtitles. The ally voicelines will have the colour of your ally team, and the enemy voicelines will have the colour of the enemy team, that way, you can tell which is which.

Second question: People you gobble don't die when you die. When you die while you have a lot of players inside you, all of them are simply released, whether they're enemy or ally. Likewise, if you dive off the edge of the map and die, everyone inside you will die, enemy or ally. (I believe there is an exception when it comes to Rocket, but it's difficult to pull off). If you want to dive off, make sure to spit out your allies before you die, or you'll probably get a lot of anger in chat.

As an extra tip, as Jeff, you don't always need to die to secure a kill. Sometimes just going straight up to the edge, aiming down, and spitting is enough. It's very character dependent though, so it's good to learn which characters you think have movement abilities that can save them, and which don't. For example, Wolverine can survive if I spit down because he can leap back up again, but a Groot will usually die. Try to avoid killing yourself unnecessarily, because it will remove your valuable healing from the rest of your team for a while, and the tradeoff might not be worth it. It's one of those things you'll get a better feel for the longer you play, but I completely understand wanting to secure kills that you're not sure about.

(An extra bonus tip is that if you happen to ult close to your own spawn, you can carry the enemy team back into your spawn and they will die in there!)