Tacet

Oh, the diagnosis is pretty easy.  The key issue in that installation is the installer. 😅 

Agreed.  I'm not an installer, and for me my installation was mostly a fun DIY project.  But my Victron installation looks neater, is certainly safer, and has been working for months now.  It doesn't cost much homework to realize that you really do want a GX. 
 
As for battery compatibility - as long as the battery can put out 48 V there's no reason for it not to work.  I'm using Narada lithiums that can't speak to Victron at all, and it works.  The drawback of not having communication is that you have very dodgy SoC readings (Victron think that mine is at 85% when it is fully charged), but that doesn't stop the inverter from inverting.

In my opinion Lithium's most important advantage - lack of fault current.  Not having to worry about whether the busbars are too close to each other is so convenient.
 

Excluding some more trunking, this is the only stuff that will be on the wall.
 
Still to be done:
Finish both systems Get CoCs for both houses Install smaller geyser elements (my little duplex has a 200 l geyser with a 4 kW element, crazy!) Install 25 A contactors to control geysers Install Sonoffs to control geyser contactors Have a Raspberry PI monitor the Venus to see when the battery is fully charged in the morning, and then switch the geyser on.

@Sidewinder
About terminology:
VRLA - Valve Regulated Lead Acid.  Simply means that it is sealed and hydrogen buildup is catered for via valves.  Not meant to be topped up, so easy maintenance, but more limited lifespan than accessible cells.  Easier to operate safely than open cells where hydrogen buildup has to be considered in the room design.  You get VRLA batteries using all flavours of inner designs: AGM, Gel.  You get them in 2 V blocks, 6 V blocks, 12 V blocks.... Flooded - you have access to the electrolyte, they're not sealed.  More maintenance is possible.  As with VRLA, you get flooded batteries that use AGM to contain the electrolyte, and you get flooded batteries that use Gel to contain the electrolyte.  
AGM - Absorbent Glass Mat.  Battery electrolytes are kept in absorbent mats between the plates.  Reduces the chances of acid spill, and offers a few other advantages in terms of performance and life expectancy.  Most prevalent design on the market. AGM is a popular design for deep cycle batteries.  Not all AGM batteries are deep cycle batteries. GEL - electrolytes are kept in a gel-like paste between the plates.  Supposed to give more cycles due to less opportunity for hydrogen to escape, but will typically be more finicky in terms of float voltage, and will generally be able to give less current than a similar sized AGM.   
Regarding lithium - why do you really want the battery to speak to the inverter?  The main reason would be to know the SoC, but why do you want to know that?  It adds convenience, but personally I don't see it as a dealbreaker.  My VRLA batteries can't tell me what their SoC is (with the exception of Northstar's ACE equipped batteries, though I've seen very few of them around), and the VRLA's silence has never bothered me.  That said, my experience is in a pure DC environment, so my requirements might differ a bit.  In theory the lithiums should give you much better cycle performance than most VRLA options, but the proof is in the pudding.  I guess we'll have a better idea in a few years.
About the 2 V cells - you do get unsealed 12 V blocks as well which you can top up when necessary, and they're generally cheaper than sealed options.  But then you have to be careful of hydrogen leakage - that's why those old exchanges had dediacted battery rooms complying to fairly stringent ventilation requirements. 

Yep, which is why the room requirements for VRLA batteries is simpler than for flooded cell batteries.  Dry-out is still the main reason for VRLA failure, though; as you said, the recombination is only effective up to a certain point.  That's where flooded cells have the advantage - they're refilled with maintenance.

Ah, so the 5000 VA model has a larger input rating.  Good to know, thanks!

Bingo, I found the answer in the Multi's user manual.  It can supply a total of 45 A, meaning that you should be fine with a load of up to about 10 kW.  That should be more than enough to cater for any inrush I'm likely to encounter in my house.

I've stumbled onto this thread a bit late.  Regarding the Huawei batteries:
If they only give 3 kWh then there's something very wrong with them.  New they should, from float to 44 V, last 118.8 minutes at a 50 A discharge current.  That is 4.356 kWh.  At the 80% end of life capacity, it should still give 3.48 kWh from a full charge.  If that battery is in a good condition you shouldn't have issues getting 4 kWh out of it.  It does sound as though these were discarded for a reason, or it was stored for a very long time without recharge.  At temperatures below 30 deg C the recharge interval for storage is 12 months, for temperatures higher than 30 degC it is 8 months.
What I don't like about them is that they have too many software functions.  You can software limit the charge- and discharge currents, which may be what happened in your case.  Huawei also have a software "lock" function which they offer the telco providers.  The lock function will basically prohibit any discharge unless the battery is conencted to a rectifier paired to it.