I had to remove the fuel rail from my 2009 S80 3.2L engine to check one of the intake injectors that was reading an error. On removal one of the end caps fell into the intake manifold and I found the ring directly under the cap is broken (not completely but cracked)

My questions
1. do I need to somehow remove the cap from the intake manifold - any issues with leaving it there? (I did remove it by using a shop vac and a rubber hose - question is more for information purpose)
2. is the black cape necessary for the injector to regulate the amount of fuel being delivered (seems to be only a cap and would interfere with the fuel injector operation) - I tried to reattach it but no longer clips on and comes off very easy all others snapped on firmly.

After testing and cleaning all the fuel injectors and after install if the retaining ring (at least is what I'm calling it as it appears is their to keep the seal in place) if I remove the fuel injector again the ring will most likely fully break and fall into the intake manifold.
3. will that ring cause any problems if left in the intake manifold?
4.. is the ring a vital component and if it does break do i need to replace it? where can I get replacement parts for the fuel injectors

appreciate any advice