What are the real benefits of having Discord on site - Discord folk are going to have their Discord app, surely, rendering this superfluous?

Discord on discord itself is way better as it has so many more features and allows for notifications. However there are many people that don't use discord so this is now a window for them.

Will people use it if it's not monetised?

Most of discord and most of the internet is used by people not seeking monetization. If your community is only built on people just seeking monetization that's I guess something for you guys to consider.

The top bar helps me see how many members there are - I'd like to be able to click on that and see an instant list of members that I can sort from there

Working on it.

Most importantly - if I post in 'communities' to the community and NOT my blog - does it appear on other front ends? I imagine this is the case until Steemit and Steempeak come out of Beta.

You can think of Hivemind communities as system to inform interfaces like us about how to organize steem posts. They're all just steem posts... but when it puts the hive tag in the first position special things happen on the interface. Also the API is really what tells us where things are supposed to go... so the hivemind system basically says put that stuff there and that other stuff over there.
So yes if a site doesn't follow communities it will continue to put it on the home pages... this is why my personal account (Hive account@jarvie) still posts some stuff with other alternate accounts because it's really meant for the community and not my blog home page.
So we sort of are waiting for everyone to come out of beta ... not sure what's going to happen with interfaces that aren't currently designing community support like busy and partiko. Do they just become irrelevant because they didn't keep up?

I'm also not sure about what will happen if I post to the community within the same 24 hour window in terms of autovotes.

Unless they specifically change their code to recognize community posts and then change their interface to give autovoters an ability to not autovote community posts then you should assume that autovote systems don't even have a way to distinguish what thing is a regular post vs a community post.

Perhaps a small explanation is necessary - something like 'View posts through Tribe pages to see your rewards in tokens specific to each Tribe'.

Yes this will increasingly be important for interfaces like us and for steem-engine itself to help users understand what it is they're doing exactly. Your definition is pretty good.

We are in search of a good description... would love to see community members or even steem-engine itself work on a good description that educates users.

For example we can have a sub-page of the community that shows all content that uses the communities steem-engine tribe token. Just another way to sort content.