I can't wrap my mind around tidbits. No way. I know of the new P/GM handbooks, are they useable by themselves?
Yes the Game Masters Guide and Players Guide, together with the Creature Guide (the three hardback books) you could play the game without cards, tokens etc. (I assume thats what you mean with tidbits?). It's also entirely possible to use the action cards as "reference cards", and not use the other bits in the game. Overall the game is very modular, you can use more or less of the bits and cards, depending on what you like.
If you plan on playing completely without the components the Players Guide, Game Masters Guide and Creature Guide are the books for you. The other products "assume" that you use the cards and bits.
Emirikol has done a great job of removing "unneccesary" components and streamlined the game into using only few of the components. I think there's a thread about it in the house rules section.
Expansion: there are so many expensions, and so focused, that they give me the impression of buying and unfinished product. Are they available without tidbits? Are useable ignoring tidbits?
The basic rules are found in the Players Guide and Game Masters Guide, and they function on their own. But the other expansions cannot be bought without the bits.
How much "ground" is covered in the PH/GM books?
All the basic rules, as well as rules for spellcasters, priests, disease, mutation etc.
I "need" some expansion?
Not per say, as I said, all the basic rules are covered in the core books. You will need to buy some packages of the custom dice for the game though.
Since you own 1st and 2nd edition you have a lot of background material and "fluff" about the warhammer world. It's quite easy to convert 1st and 2nd edition "on the fly" to 3ed (I've done several 1st/2nd edition adventures and just converted difficulties and such at the table while GM-ing, WFRP 3ed has an easy enough system to allow quick conversion like that.
Some of the new adventures might also be interesting for you, but you certainly don't need them to play.
Some final words:
I don't think 3ed should be played completely without components, the game works better with at least a protion of all the bits (such as action cards etc.) even if it's possible to play without any components. I'd recommend you to try with components a few times and then decide if you like it or not. I was sceptical at first, but now I love all the bits and cards.