question

The Multi with a grid meter installed between the grid and your loads on the AC input side of the Multi will be powered from the multi as long as the grid is present. But if the grid goes down, the Multi will disconnect from the AC input and only power loads connected to it's output.

You are getting the story bit by bit :)
As Alexandra says you have to have a battery bank with the Multiplus 2, which you do!

As Kevin says if you have your loads and your existing PV on the AC input side and connect the grid to your AC IN on the multi it will work (AC IN on the multi is bi-directional in an ESS configuration with control of the AC in/out current as a configuration option).

However, as Kevin also says in the event of a power outage the Multi 2 will disconnect when it loses the grid feed, plus your existing (grid tied) PV will stop working. The way to get around this is two-fold....
First you must have an isolator switch at your grid entry point so that there is no possibility of you feeding back into the grid (to protect engineers working on the line). Second you need a changeover switch at your Multi to switch the AC IN to AC OUT1 (it is important to switch both live and neutral in a break-before-make switch) where the common of the switch is your "grid" connection, probably via a consumer unit MCB?. Now, you will be off-grid, the Multiplus inverter will supply AC to your consumer unit for as long as the battery holds up, plus your existing PV should be "fooled" into thinking it has grid power and feed in to your house as normal (if the sun shines).

Why go to all that complication? It will probably be easier and require fewer components if the OP rewires his existing loads and PV inverter so they are permanently on AC-Out.

If you are going to fit a changeover switch in case the Multi develops a fault the common terminals should be connected to the loads and the alternative inputs are then the incoming grid and AC-Out. Then you do not need the isolator (though it is still a good idea) and there is no possibility of inadvertently backfeeding the grid as the relays in the Multi will disconnect it automatically.

The easiest way to add a Multiplus to an existing home wiring system is to only use AC IN in an ESS set up(particularly when the Multiplus is not sited close to grid entry point). The OP was asking "will it work when i just connect in with AC-IN?" I was merely pointing out that this will only work when grid power is maintained since the multi will disconnect during a power outage.
So, I agree with you, starting with a clean sheet it would be possible to use AC OUT 1, separate critical and non critical loads and wire the house accordingly. However, from experience this can be an expensive and/or extensive re-wiring job. My "suggestion" (and it is only that) of a changeover switch on the input side is to minimise re-wiring/load distribution and to make it impossible to connect AC IN to AC OUT (via any wiring route) at the Multi, since the house wiring can only be connected either to AC IN or AC OUT but never both.

Maybe we are saying the same thing, your statement "where the common of the switch is your "grid" connection" is a bit ambiguous.

I agree if the inverter location has to be distant from the grid intake then wiring the house load to AC-Out will involve running two extra cores (but is that such a big deal?)

IMHO doing this gives a much better configuration: in a blackout the changeover to EPS mode is automatic and unnoticeable and your PV will still work.

The OP is talking about a 5k Multi; the 50A passthrough means it is probably enough for all but the largest houses (unless it has electric space heating) and without having to separate out non-critical loads.

You shouldn't just hang the AC-In of the Multi off a spare way in an existing CU anyway, it will work technically but doesn't meet the requirement that the EESS has a separate RCD. So once you add a mini-CU for the inverter you might as well rewire the existing one to AC-Out and bob's your uncle.