Andrew, do you have a way to check your pH? My first thought is that the pH got too low, resulting in the crumbly texture and tangy / cheddary taste.
For growing the PS, my experience is that 20°C (68°F) is definitely way too cool, and even 25°C (77°F) may not be warm enough. Even in the summer, when the house stays around 78°F, I find that I need more warmth to get swelling, at least with an Emmental-style make. I put them (in a ripening box) on top of the fridge, where some warmth from the coils raises the temperature a bit higher.
One other thought - PS is sensitive to salt; if I recall correctly, salt levels should be < 2%.
I've never read of any need for LH in order for PS to grow, and in fact my first Emmental-type makes did not include any LH, only ST. The LH does add some nuttiness to the flavor, and is worth includinig, IMHO.