[Ted S]

Great question YabbaDabba. I'll do my best to explain...

As you know notices are "native" to 3.7 & 3.8 and for most people who just do basic messages they fit the bill.

Of course as you've discovered the system is pretty basic and while there's a lot of options it actually doesn't do everything the simple conditions in welcome headers did (on the other hand it does a lot of cool things welcome headers did not do).

Yes, both can be used at once.

Yes, the 3.6 version works on 3.7+. No, nothing has to be tweaked to do this...

I personally run the 3.6 version on my client sites as it lets me build whatever conditions I like using template variables rather than relying on what is in the notifications system.

As for your last question the difference is pretty much code vs interface. vBulletin provides notices to let people build their own messages -- welcome headers uses template conditions to do the same thing. With my 3.7 mod you get a set of notices using the built in feature, with the 3.6 mod you get a template that identifies the right scenario to display a message to.

That solve your questions?