Your first link seems to be 349€ for 800Wp including microinverter. German utility-scale PV has a capacity factor averaging about 12% IIRC, but presumably a balcony system will be lower because it isn't optimally angled for the sun, say 8%. Then that's about 64W, which is 560 kWh per year. 349€ over 10 years would be 35€ per year, about 0.06€/kWh.
64 watts is about 40–50 liters per day of hot water heated resistively, presumably closer to 150 liters per day with a heat pump. But it seems like the heat pump is only saving you the 700€ for two more such balcony systems, assuming you have the space. Moreover, you don't need a microinverter for a resistive heater.
Oh indeed, balcony systems are small and limited — the constraints that led to them are: (1) DIY-safe, (2) useful for rented apartments (limited window or balcony fencing space, may not be allowed to fix something to an exterior wall).
I'm not sure if you're allowed to just resistively dump an off-grid PV system into a resistive heating system, but I guess if you did, you could indeed save on the cost of the inverter.
efficiency of no MPPT is around 10 percent lower then with MPPT system so no issue. 2000 watt is maybe max what you can have on balcony - physical size, and 2000 watt inverters are like 20 $. well connecting anything to grid is/should issue, offgrid noone cares.
You might be able to hang an additional balcony system out a non-balcony window or something, or just keep it inside the window. Less insolation, but maybe not so much less that it's unprofitable.
I believe that even in Germany you can connect a low-voltage solar panel to a battery or heating element without any licensing approvals beyond CE. Low-voltage wiring poses much less risk of electrical shock and is consequently exempt from most safety regulations.
That's still about six times the cost of wholesale low-cost solar panels: https://www.solarserver.de/photovoltaik-preis-pv-modul-preis...
64 watts is about 40–50 liters per day of hot water heated resistively, presumably closer to 150 liters per day with a heat pump. But it seems like the heat pump is only saving you the 700€ for two more such balcony systems, assuming you have the space. Moreover, you don't need a microinverter for a resistive heater.