I understand that this is one item that can be transferred into BG2, and I assume that is still the case when playing with BGT. However, getting it is part of a quest. I've done that, but it seems impossible to keep the darn thing while completing the quest. The chap that gets it vanishes into thin air before you can blink. Is there a way, or must I simply not go back to him with it, or maybe kill him on site?

You'd have to kill him extremely quickly. Surround him with fighters, use wands, potions of fire breath - anything to drop him fast.

The XP whore in me shudders in disbelief and soulcrushing horror, but I guess that's the only solution.
(Okay, we went back and killed the guy, he didn't seem very nice anyway).

A different issue, is that Firebead seems to have got a bit of a walking problem. Went looking through Beregost because I remember there was a guy *somewhere* wanting a book, and finally found him. But he kind of twitches around a bit, walking towards us, but without initiating dialogue, and I can't initiate it with him either. Either doing it so late has caused an issue, or something else is messed up somewhere. It doesn't work, at least not in BGT. The game autopauses when those 'friends' of his show up, I order all of us to attack him (need to force attack as default action is talk), but the instant I unpause the game he simply vanishes into thin air on the spot. I'm guessing there has been a script change so it's now impossible to kill him to get the helmet.

So instead I decided to attack him without dialogue. Bit odd his friends didn't show up, but he died and we retained the helmet.

Stand next to him, save your game and then reload. Not being able to interact with npcs and objects is a known issue with several game engines, IE included. Saving and reloading generally 'fixes' it.

Actually, you don't need to bother with all this.

According to
http://baldursgate.wikia.com/wiki/Importing_a_Character

all you need to make sure that you don't have any of these items when you transfer the character:
Claw of Kazgorath
Horn of Kazgorath
Koveras' Ring of Protection +1

Yeah, but why would anybody *not* retrieve the very best helmet in the entire game?

I tried this when going back to Beregost (and Gerde who just keeps saying we don't have the stomach, despite killing probably 20+ ankhegs), but nothing changed. No biggie though.

That's not the issue with Gerde. It's a well known, long standing bug in the Gerde/Ankheg quest to do with variables not being set correctly. It wasn't fixed in BGT, and if I remember correctly it even transferred to the enhanced edition -- not sure if they fixed it.

In BGEE I think the counter only works with Ankhegs that are fixed encounters on the map (encounters that appear at a specific location at a specific time of day). So spawns and rest encounters don't count towards your kill count. That may be how the original and BGT handle it as well. Not sure.