1) Is a clogged orifice a higher frequency failure when compared to a stuck solenoid?
I wouldnt say either are high frequency but I've had a clogged orifice once on my mCCR. I've known 3 people that have had stuck open solenoid valves. (2 stuck wide open on a dive and one was partial but slowly letting more O2 into the rebreather and spiking the ppO2). Both happen more often in salt water for sure.
In my case it was not a complete clog as oxygen was still being delivered I was just having to manually add oxygen more frequently than usual. I noticed it over 2 dives and when I measured it with a flow meter my typical flow rate was almost cut in half.
Smaller orifices are more prune to clogging. I believe most mCCR also have a micron filter to help prevent clogs.
I dont own one but one of the supposed benefits of a needle valve is that you can, in theory, open the orifice large enough to dislodge a clog that might occur. You can't do this on a fixed orifice like on the rEvo or KISS.
2) Is a clogged orifice something that every mccr user is expected to go through at some point or another (say over the course of 10-15 years)?
I think there are a lot of variables such as how many hours on your unit, salt water intrusion and your cleaning procedures. I know a lot of mCCR divers that have never had a clogged orifice. Part of your training (and in some cases build procedures) should be to check oxygen flow against a flow meter and verify the correct flow rate. It is something you train for and should notice.
3) If the ccr is used for cave diving with avg depths of 20-30m, what would you rather have: a clogged orifice or a stuck solenoid (i.e which one is less dangerous)?
A stuck open solenoid in my opinion is vastly more dangerous. It can introduce a very large spike of oxygen quickly and send your ppO2 sky rocketing. It is very much a situation where you need to react quickly and shut down your O2 valve. This is effectively an internal O2 "boom" which is something also covered in your training. At this point your rebreather is effectively not usable as it was designed, you would need to feather your O2 valve, isolate the solenoid, or even bailout depending on your training and experience.
On mCCR a partially clogged orifice just means you need to manually add O2 more often and is merely an inconvenience. It's still a mCCR and still functions as such. Even completely clogged it will still function as a mCCR and the unit could be "flown" manually albeit it would be annoying as you have to add O2 much more often. It doesnt mean it's not dangerous. One of the golden rules of rebreather diving (mCCR or eCCR) is to always know your ppO2. There are people that have died from O2 shutoffs on mCCR (effectively the same thing as a fully clogged orifice) when they did not monitor their ppO2 and incorrectly assumed their rebreather was still delivering oxygen when it was not.
4) Anything else I should have asked but didn't?
I dont think neither are inherently safer. Both are just failure modes to be made aware of. Whether you choose an eCCR or mCCR both of these scenarios are something you should train for and know how to check/test. Both are dangerous if proper measures are not taken.
A fully stuck open solenoid can lead to hyperoxia and sudden change in buoyancy if not caught quick enough.
A fully stuck closed solenoid (their designed failure mode) can lead to hypoxia if not caught quick enough
A fully clogged orifice on an mCCR can lead to hypoxia if not caught quick enough
Both scenarios can result in death. Hence the golden rule here: Always know your ppO2.
