Does someone with shoulder impingement/tendonitis become susceptible to a possible RTC tear?

Spot on Sheena! Because shoulder impingement is either due structural or non structural cause, the supraspinatus tendon will become irritated and ultimately microtears occur in the tendinous sheath. Making it susceptible to turning into a tear due to the stress received in the frontal plane.

That is why it is important to reduce the load(stretch the posterior capsule), stretch the pecs which will biomechanically unload the front of the shoulder and avoid forward raises(anterior deltoid) and chest press which will only compress only create more impinging on the tendon...