question

1. tldr: Yes.

If you want to install inverter parallel to the grid then yes. However if you want power when grid goes down you'd need to configure the "critical loads" on AC out rather than ACIN1 (which i think what you meant by UPS comment is that you dont want this). However if you do and if your max loads aren't huge or your multi is large enough. It might be better to make all the loads "critical". ie rather than grid and loads parallel to inverter using AC in. You use Grid -> inverter via AC in -> AC loads/consumer unit connect via AC out.

2. Yes you use VEconfig to setup ESS on multiplus connected to PC or alternatively you can do this remotely via VRM if multiplus is setup with cerbo GX or raspberry pi. I'm not sure but you may need GX device for ESS but if you have GX version of multi then that's built in, otherwise might need a raspberry pi or cerbos. Someone can correct me on that.

3. Yes, the connection is RS485 so you can use a standard cat6 ethernet cable to the connect from Et112 to Multi, if the cable length is long you might need to add a resistor or go down the route of radio/zwave as you mention. I needed to send mine a long way so i used RS485 to TCP/ip server client to send the data over the internet down to my shed. It adds latency but worked for my scenario, due to latency it means about 2% of my usage still comes from grid. Of course if meter is close to multi then just cat 6 wire is best.

4. You connect to the GX device i mentioned in question 2 (either raspberry pi, built into multiplus or cerbos GX) via a browser (ie using internal ip such as 192.168.1.xx). Go into settings -> ESS then you'll see schedule charging. You can set time and max level to charge to etc in there. If you require more advanced tuning you can use Node Red however it sounds like the built in schedule charging will be ideal for your needs.

Thank you so much. This is excellent information. Yes point 1. I am not planning to use the system to take over from the grid, just to supplement/replace it whenever possible - battery charge permitting.

The units I have are the non-GX multiplus II 5000. I had no idea raspberry pi could take the place of cerbo gx, I am quite familiar with raspberry pi, so I will look into this before buying £300 cerbo gx units (I am doing three installations).

Thank for your time.

I will report back on how I get on, for the benefit of other users.

It's so much cheaper, it's amazing assuming you can get hold of a raspi with the current markets. I'm not 100% sure if tanks sensors work with raspi but standard stuff raspi can do instead of needing a cerbo.

Just a heads up there's revisions of raspi 4 that aren't supported yet. ie if you've got an old raspi 4 you'll be fine but the later versions they dont work with stable release of venus os. (I wouldnt worry though, i've got a raspi 4 that isnt supported yet but I'm running the beta version of the venus os with no issues/bugs at the moment with a similar setup to yourself. Venus os 2.9 once released should support all raspi 4s, currently this is in beta). This raspi 3 doesnt have any issues, if you have one of those lying around.

I also wired grid parallel similar to you (as 5000 multi isnt enough for 100% of my loads). I've added a disconnect switch on the grid and then a transfer switch at the inverter so that if the grid does go down i can flip the two switches and go in off grid mode as just avoid heavy loads.

Of course it's not as clean as the multi doing the switching but I couldnt afford(/justify) going bigger with the multiplus just for that feature.

Raspberry Pis work (Have 2) but availability and price if you can find one are prohibitive at the moment. If you're looking at using Interface Mk3 USB and VE.Direct to USB you will need for the PI will likely add up to almost the price of Cerbo GX.

Additionally the PI is only a community supported solution where as Cerbo is fully Victron supported. On a production/critical system I personally wouldn't use a PI

Just my thoughts