If the downstairs unit (fan?) is off but the system is still sensing refrigerant to it, then you have a major problem! If they can't fix it, then they need to rip it out and start over with two separate, correctly-sized systems. 99% of installers have no clue how to set up zoned systems and minisplits just add to this dire situation.

The condensation from the mini split head is supposed to drain to the outdoors. The head is usually mounted on an exterior wall and the drain line is designed to go straight out with a downward pitch. In your case the drain line needs to traverse an interior wall and make its way to the outdoors or some drainage point. The drain line has not been installed correctly and the condensate is overflowing onto the floor. This drainage problem is independent of having one or two outdoor condensers in my opinion.

I am not a mini split expert, but the head should not be making popping and cracking signs. Were you given an option installing one versus two outdoor condensers?

While we can't rule out a problem with the drain line, the OP relates that the evaporator is freezing up. If the coil freezes up, melting ice can put water outside the drain pan's ability to catch it. Ditto for the condensation which is evident on the exterior of the indoor unit; that will drip onto the floor.

It appears there are several serious problems and the installer has not been able to resolve them in 7 attempts.

It seems to be condensation and drain pipe is likely fine IMO (at least we want to start with that freezing up while off as the primary issue)

We had a lengthy discussion several months ago about a homeowner complaining about mini split head installed on an interior wall and condensation dripping on the floor below it. It is something that should be investigated.

Unlike the post mike_home referred to above, the OP's condensation problems are not due to condensate produced as a result of normal dehumidification in air conditioning mode. You can see condensation on the outside of the unit which is not normal; nor is freezing of the indoor coil. For a multi-head/multi cassette mini-split system, refrigerant should not be delivered to a zone not calling for it. I'd check to make sure the plumbing and control wiring are installed correctly, then I'd check for a problem valve or a control issue.

The company which installed the unit should have access to troubleshooting help either direct from the factory or through their local distributor. Sounds like they'll need it.

MnM 01 we are experiencing something similar...do you have any more insight on what the wiring issue was? Not sure if you were experiencing the same but when our downstairs unit stops working (since it froze up) the upstairs unit stops providing cool air as well. We have to wait until the downstairs unit defrosts before anything starts working again.

I am not sure the exact issue, but it should not call for coolant unless on and should not be freezing over, when it freezes over like that, it has a safety I think that sends a signal to stop sending coolant so that is why it likely shuts both down. Get someone that knows how to fix it out ASAP before damaging it.

Is the unit that is freezing over off or on when it freezes?