1) first time playing (red ship), I made it to the little girl and her many different forms (kicking all of their asses). Is this the TLB? Amazing that I 1CC'd on my very first try!
Yes, that is the TLB. As long as the timer in the bottom-right doesn't hit 0 before the final stage, you'll get the TLB.
2) Second time I played the yellow ship and had some differences: I could choose 1 stage after the first; the rest played out normally. Are there other stage paths, and how do I trigger them? Is it dependent on ship selection?
There's a stage choice for stages 2-5. You might have to unlock the branches, I don't remember. There's a number of unlockables in the game, from starting rank to ship choices to bullet colors and more, and they're all unlocked simply by playing the game a bunch. Things unlock quickly at the beginning but it slows down later. I've heard but haven't verified that it's dependent on how many games you've started regardless of how far you get in each of them. Speaking of ship selection, there's an alternate variation of each of the six ships you see in the ship selection screen - I missed that for a while since there's no indication when you've unlocked the alternate types, you just become able to scroll past the first six ships or up/down.
3) I killed the teddy bear as the yellow ship, but it killed me after its death (I still had lives in stock). It said I had the "and mankind killed by a teddybear" ending and all that - is this intentional or did I fuck up?
I think that's just the non-TLB ending. Never got that ending myself, though, except on my first playthrough of the freeware version which wasn't translated.
4) Does speed-killing up rank, and is rank tied to firepower? It's not immediately clear to me.
Getting the time bonuses on bosses and minibosses increases rank, but I don't think speed-kills increase rank otherwise. In the original freeware version of the game, rank was directly tied to firepower - the higher your rank, the greater your firepower. In Steel Rondo, rank still affects firepower but there are other factors at play and I'm unclear on exactly how it works.
You know STGs are in trouble when you have threads on how to introduce them to a wider audience and get more people playing followed by threads on how to get its hardcore fan base to play them, too.
1cc list | Twitch | YouTube