@solonger I’m not a fitter nor a MD/medical expert but, I’ve made every mistake in the book cycling wise. Twice! So, the quadricep tendonitis seems consistent with a saddle slightly low. You said you switched shoes and it felt high so you moved the saddle down. Did you move the saddle down after or before the pain started because maybe you’re too low.
The posterior tibial tendon issue I’ve not had but, some of the symptoms are similar to plantar fasciitis which I have had. For me the plantar fasciitis was caused by a saddle too high. Moving it down and stretching the hell out of my calf resolved that issue relatively quickly. Not saying this is you just that trying to piece the clues together have you ruled out plantar fasciitis? from what I understand it’s more common to develop this with a saddle too high however, if a rider ankles deep enough with a saddle too low, I could see how the same symptoms could arise.
Lastly, cleats can be tricky tricky tricky. Even with SPD-SL yellows (float) if mine are not aligned reasonably close and/or they move forward for lack of me checking my knees will feel it first.
Now that I’v confused you I’m going riding! Hope you get it sorted soon…Cheers 