No,

The ELB 500 TTL head is not compatible with the Quadra / ELB 400 Unit.

The Quadra heads cannot be used on ELB 500 TTL units either, an error message will appear on the screen.

The voltages of the packs and heads are different and cannot be mixed up.

We are in a world where we need to work faster with an efficient workflow.
This is why we introduced TTL within our portfolio of products. 

Yes, it works seamlessly.

Read more about this.

Previously, we only introduced products with Hi-Sync as HSS had a substantial disadvantage of power. 

With the ELB 500 TTL, we have been able to optimise HSS to the point that it is not dissimilar in power to Hi-Sync, thus making Elinchrom's HSS the most powerful HSS from comparable powered units on the market to date.

No.
We decided to only implement HSS as it offers a good compromise between flexibility and performance.

TTL is only here to help you if needed.
So yes, if you don't want TTL, you can just as easily use it in manual.
If you want to start in TTL, lock exposure and then switch to manual, this is also possible.

The heads and batteries are not compatible. 

All Q-Mount accessories; light shapers, protective caps and cables are compatible with the ELB 500 TTL.

It's a feature that enables you to charge while using the unit, this means you could keep the modelling lamp on at all time.

The ELB 400 with a Quadra HS head and the ELB 500 TTL with an ELB 500 head are so very similar from power output. Whether one is more powerful than the other now depends upon the individual camera. 

HSS offers the advantage of less exposure banding and no gradient in the exposure, than HS.

The ELB 500 TTL offers 100 Ws more power, a new easy to use and large interface, TTL, HSS, full asymmetry and to do so a completely new internal circuitry.

The ELB 500 TTL also offers only one type of head to shoot all kinds of various photography possible.