Paul, if it has been that way for a couple of months, and the location of the gaps hasn't shifted, then I'd be fairly certain that it's a clog. Moreover, and being completely honest, I don't like your chances, given the duration of that clog. These things are best dealt with promptly.
I take it that you've seen the IJM small printer cleaning video? The only thing that I think has a chance of working at this stage is the cleaning kit with the little syringe and dummy cartridge and piezoflush. A word of warning. With a stubborn clog it's tempting to push really hard, but the dividers between the ink channels inside the print head are thin, and people have ruptured them by pushing too hard. How hard is too hard? Dunno.
That said, at this stage you don't have a lot to lose, and in a K6 R1410 that I once had with a totally blocked black channel (due to neglect) , I pushed and pushed really hard and didn't do any damage, but also I didn't clear the clog, which may be why I didn't do any damage.
Re Windex (the version with ammonia, which is hard to find in this country now), I really don't know. IIRC, the IJM position is that a fast-drying solvent like Windex risks damaging the print head. Out on the wider internet flame wars have been fought over the issue. I used to use it, but don't any longer. I don't think I did any damage, but honestly, if you had a printer that eventually died after some years, how would you know whether it was the Windex or old age? I don't use it any more because I don't like the smell and piezoflush is not that expensive if you buy in bulk at the same time as ordering ink.
Re over-thinking and that gut feeling, when you have shifting gaps in the nozzle check, it can be mighty hard to make an educated guess as to what the precise cause is. Is it air in the system? A slowly leaking cartridge, and in the case of piezo, which one? A dirty or faulty capping station? A printer that doesn't like refillables? IJM have been quick to blame the capping station and absolve the cartrige, but I have found cartridges to be the problem in many cases. So I am not surprised that IJM have a completely new cartridge design.
My general advice is that patience and purge patterns are better than head cleans. Patience can be hard, but waiting a day allows air to settle and the brain to think. If the gaps shift locations, then head cleans can cause more problems than they solve. Of course, even this rule does have exceptions. E.g. if you have a channel that is barely working with only a couple of nozzles printing, and they shift, but the number of working nozzles doesn't increase much, then my experience is that a head clean is needed to pull the ink through. But this doesn't apply in your current case.
Glad to hear that you included a spare bulb.