Hello,
Sorry for the late reply, but I didn’t have time earlier. The heating and cooling system issue has been resolved. I installed a new heater core from NRF. For the old elbow/quick connector, I managed to find an O-ring with dimensions 3.5 mm thickness and 25 mm inner diameter. Ultimately, for a reasonable price, I bought a hose with an elbow from Gates (05-2567) and installed a new one. I lubricated the O-ring with a small amount of ATE paste for brake cylinders (a friend advised me). Thanks to that, the installation went smoothly. Before replacing the heater core, I used Kleen-Flo flushing fluid for the cooling system; the car ran on it for 10 minutes. I drained the coolant and replaced the heater core. Then, I flushed the system twice with distilled water, and afterward, I poured the final G12/G12+ coolant. There are probably still trace amounts of water in the system. As you suggested, the problem was the heater core. With the new one, I managed to bleed the cooling system very quickly. Now you can really feel the hot air when I turn the heating knob. After some time, I will remove the timing belt cover to check if everything is okay—whether the belt is not slipping and how it looks. Thanks to everyone who contributed in this topic. Your help was invaluable.