You still have that M/Y printhead? What specifically happened to it? Was there an error code?
Heads generally die for one reason: they get air in them over time until eventually they are trying to print with air instead of ink and the thermal head burns out (which will have an error code). Usually you encounter print quality issues long before that to know it's happening. Or can weigh the heads -- 60 g is full, and ~45 g is empty. If there's just print qual issues and no error code, then most likely the head is just low on ink in 1 or both colours.
You can purge the air from heads and re-prime them with ink and they run again like new. If we had all known this years ago we could have saved $$$!!! on heads replacements (and burning ink / paper in the process). But heads were cheaper and easier to get, replacing was less work. And I think we didn't know then what we know now. The head shortage / discontinuation has bred innovation.
So if you implement some practices you can keep your heads going a very long time, and restore some that "stopped working" previously to have as spares. (Or if you do decide to dump the printer, I'll take them haha!) But the best thing you can do right now is remove the heads to weigh them (and clean them while at it).
As for the black spots, a photo would help diagnose because it depends on the location on the print and shape. But more than likely there is simply some crud on the underside of the printheads and carriage, and also built up around the service station on the right side. It builds up over time, especially in printers that sit idle. It's easy to clean tho. So it's not the head itself.
