I'm no expert on biomechanics, but from my 'back seat adviser' position.
You've clearly got a 'problem' with your running which needs resolving. I know there are some posters on here who regard osteopaths and chiropracters as purveyors of snake oil, but the one you visited prescribed something which (s)he thought would resolve the problem. You seem (and apologies if this comes across as harsh) to have bailed on the resolution as it hurt a bit when trying to correct your default style.
Your physio is keeping you going, but it doesn't seem she's fixing the basic problem. Are there not strengthening exercises or stretches which will stop the stiffening?
If, at the end of the day you can afford the cost of the physio then the status quo can continue, but you are going to have to up the mileage at some point and if you've not fixed the root cause then you'll either break or need to move in with the physio.
Certainly this far out from the marathon there's no rush to start a specific programme so you've got loads of time on your side. I'm a big fan of core work and certainly shedding excess weight is good, but I'm concerned that you're not addressing the core problem. What is it that your physio does every week (is she doing something you can't do)? What was it that the insoles were doing to make things worse?