Wow, great post 2Bit_Hack!
Some quick answers.
MentalMars has already answered this, but to confirm, our plan is to support a full retail launch. It will be a veritable deluge of content for you guys. What happens after that largely depends on you guys. How much do you like and play the game? With Borderlands, we offered a lot of DLC content, and different types of DLC content. I’d love us to do that and more with Battleborn, but that’s the discussion we get to have after we get the initial content in great shape.
Steam. Similar to Borderlands, it lets us integrate your steam friends list and such to make it as easy as possible to party up and find people to play with.
Our Story Mission/Episode/Quest/Things, which each unique with story and campaign content, are being designed around ‘mode’ concepts. Several of them feature horde-themed gameplay which should give you the experience you describe here. Thinking about them like game modes also makes them interesting for replay…it’s not quite the same experience every time. None of this content is being designed to “fire once and forget”. I’m hoping we all love to come back and replay many times to try different characters, unlock new rewards, and face different challenges of play.
That’d be cool wouldn’t it. 
Marquis has Hoodini his owl. It’s not quite the pet class you’re talking about, but there’s a little of that vibe there.
Also there’s XXXXXXXXXXXXXX, which will probably give you more of the pet play vibe than that.
Iiiiiinnnnteresting. I had not quite thought of “Spores” like that, but you might be right. Something to think on.
Oh yeah. The paint’s still very wet on this one. I’m sure that’s not the only type.
Also, for you and Mr. MentalMars both - those numbers on the website? Take those with a grain of salt. They are by no means final yet.