Please help interpret the manual:
“If voltage is present on both inputs, the Quattro selects the AC-in-1 input,” and
“The Quattro operates in parallel with the generating set OR the shore connection”.
This means the quattro will not blend AC1 and AC2 inputs. Only AC1 + battery OR AC2 + battery. Right?
Maybe I should say “combine inputs”
Correct. Quattro does not blend two AC sources. Can’t… how in the world is it going to synchronise them? 
The difference between the Quattro and the Multi, is that the Quattro has two (interlocked) transfer switches. Only one of them can be active. AC-input-1 has priority, so if both have (acceptable) power it will pick the first one.
Also, perhaps not so clearly in the manual, but when the battery is completely dead, a Quattro can power itself from AC-in-1, but not from AC-in-2. That means that if you need to do a black-start, you cannot do it from AC-in-2. It is therefore adviseable to put the genset on the first input.
Read somewhere that you get an AC Combiner (for micro-inverters, I think). How is sync established in such a device? Sorry, have a whole store of stupid questions.
Wouldn’t the micro inverters actually sync to the house’s AC? They aren’t a source of AC.
You’re probably thinking of the “Gateway” device just about all of them have. That’s just for monitoring and logging data to whatever cloud solution goes with it
Micro-inverters are a kind of “grid tied” or PV inverter. They cannot work on their own. They simply take the power they produce and push it straight into the grid. To do this, they sync with the grid. They use an electronic arrangement known as a PLL (phase locked loop). They all sync with the grid signal, not to each other. In fact, they are designed to deliberately fall out of sync when they are on their own, to prevent accidentally making a powered island in a sea of dead grid (which can be dangerous).
You made it so clear. Thank you.
So for micro-inverters to integrate with a backup solution and not feed into the grid they must be connected to the AC output of a Multiplus (for example) so that they have something to sync with. I now get why setup is called a “microgrid” in Fronius terminology. In this topology is there any way to use solar to charge a battery apart from a separate string as DC input on the Multiplus? I suppose that you cannot add a charger to the AC output of an inverter without creating a loop - using the battery to charge the battery when the mains is down?
Yes and No. Any PV string inverter you are likely to come across will have the ability to frequency shift. That is a smooth reduction of output power as the upstream hybrid inverter raises the frequency in off-grid mode.
Not all microinverters are capable of this ( I think Hoymiles is). They don’t have a smooth curtailment of power, just grid rejection. They are either full on or full off. So confirm this functionality first.
Yes, per my understanding (I don’t have such a system) the Multi can rectify the AC from the PV inverter into the batteries, if it has spare capacity.
Correct. Think of the Multi/Quattro (@pula, Multi is short for Multiplus, and the Quattro is the same except it has an extra AC input, so we will often just say “Multi” for short) as a synchronous rectifier that puts the battery in parallel with the grid. By adjust the power levels, it can then allow energy to flow from the AC side to the battery, or from the battery to the AC side. If you tie a PV-inverter to the output of the Multi (and you configure it to operate in a Microgrid), the Multi can indeed accept AC power on its output and charge the batteries with it.
Johannes explains some of it here:
