When not tied to grid frequency, the output could be deliberately designed to drift high or low, so it is out of bounds, and switch off.
I think we would need a rotating load, like a grinding wheel running to smooth the supply enough for that to happen, but clearly we could have a rotating load which could do that. But still nothing to do with being plug in or installed.
Well, I suppose it depends on the relative 'shut-down speeds' of the two or more inverters - but, as you say, the same would apply to any PV installations, whether 'plugged in' or not.
So what we are saying is, there needs to be a standard which must be complied with, and it does not matter if plugged in or hard-wired.
So an installed inverter has dip switch options

where it is set for the country where it is being used. This is why the installer has to be trained, and licensed in some way. So to be used in the UK, we will need some sign that they are configured for UK use, be it a UKCA label or some British standard, something will need to be written on the unit to show safe for use in the UK. And I would hope the options can't be set by the user, or someone is sure to fiddle with them.
In the early days of solar, we saw this, mainly with the rent a roof system, if the inverter is designed to cut out at 253 volts, and by altering the setting you can make it trip at 254 volts, then you can ensure everyone else solar inverter trips before yours, so you get more solar than your neighbours.
I have even seen a video of someone using an auto transformer to kid the system that the voltage is lower than it is. And this guy should have known better, he has been very quick in the past to slate off other peoples work, then he no only did this, but made a YouTube video about doing it.
And then the DNO actually paid for the single phase supply to be swapped to three-phase, as they had accepted the G99 in error, I would have expected to see the guy's licence revoked even if the problem was due to a DNO error, two wrongs do not make a right.
So this has happened with electricians, what will happen when Joe Public gets involved? The only way is for the inverter to be a sealed unit, that the seller has signed to say it complies. And since we as yet don't know what rules it must comply with, no supplier can build up a stock, as we simply don't know what will be permitted.
With early solar, the output was not monitored, it was assumed a percentage of the generation would be exported, and they were paid on that assumption. Then we go things like the iboost+ which were designed to let the householder use a larger percentage of the solar themselves.
The question has to be, will we also see this with plug and play solar, some device marketed to get around the design parameters? As to how we will PAT test these devices, not sure, it is in service electrical equipment so should be inspected and tested in any rented property. But as it stands, only the landlord is forced to inspect and test, items owned by the tenant don't by law require inspecting and testing.
So as a tenant who has jumped the gun, and bought plug and play solar panels today, which will likely even if they comply with the electrical requirements, will not have any insignia to show they comply, will they still use the units anyway? I would think they will, as it stands, if next door gets plug and play, the supplier will see at some point some export, so they will know that since no G98 or G99 issued, they are breaking the rules, solar panels shown on Google Earth may not be grid tied in and illegal way, so that does not help, so as it stands, I could fit plug and play, and since already have a G99, no one without visiting the home could tell if I have broken any rules or laws.
Since whole idea of plug and play is tenants can use them and take them home to home, the whole idea of applying for a G98 goes out of the window, if that is required, they are not plug and play.