That is quite a concern: I'm happy to not use source where it's optional, but ISTR there's a couple of places where it's actually quite necessary (such as removing the barrier in Braccus' tomb).
That's actually not true, the only thing you'd miss if you'd never removed your collar in act 1 is activating the shrine to get the helmet.
The barrier under the Gargoyle's Maze can be opened without using source. If you liberated Withermoore from his curse earlier in the game and touch the mysterious mechanism with whoever agreed to help him, he will appear to help you through.
