Just a few thoughts and comments from an experienced coldwater diver in the hope that it will provoke some serious discussion between you and your buddy.
Firstly, congrats on getting back okay.
Yes, it is advisable to have sealed regs when diving in cold water. However, even sealed regs can malfunction.
Question - on your dive did either or both of you stick your reg in your mouth before entering the water?
It is very wise to wait until you are under the water before you take your first breath from your reg. This is to avoid the reg's "freezing up" at depth by your breathing in moisture that subsequently ices up, very often in the first stage, at colder depths.
Question - you say your buddy's reg was "studdering" - did he have his air valve turned on fully before the dive? If the valve is not fully turned on then it will not deliver air efficiently.
Interesting that your buddy's perceived solution to the freeflow was to seek escape to the surface.
He asks to share air - prompting a rescue scenario - but is unwilling to "submit" to the rescuer's request to return to the mooring line.
This is entirely understandable of course. How can he have full confidence in a buddy diver that has not been rescue-trained?
Many dual fatalities have occurred because of unsuccessful rescue attempts. One major contributor to tragedy is when the rescuer is denied the control needed to successfully carry out the rescue.
In your case your buddy was the "senior" and more experienced/qualified diver in the incident. He could have, and maybe should have, adopted the role of rescuer even though he was breathing your air.
Another major contributor to failed rescues is when the ascent begins too soon. That is before a comfortable breathing rhythm has been established.
If there is plenty air at hand, as there was in your situation, then use it! Take one minute, maybe holding on to that mast, to calm the breathing of both divers and exchange signals. Then, when buoyancy of both parties is neutral the ascent can begin.
An escape to the surface is often the wrong immediate choice.
The third diver's pony - kudos to the donor, but, yes, I would have to agree that it was unwise to accept what might have been a "rich mix" from an unknown source. Even though it is extremely unlikely, given the short breathing time allowed by the pony's capacity, that any toxicity issue would have occurred.
I would hasten to add that I believe you would have been better prepared to enjoy the 100ft dive had you done a couple of work-up dives beforehand.
In cold water the onset of narcosis can occur as shallow as 80ft, just enough to increase the stress factor.
Whenever you are surrounded by bubbles on an ascent you are passing them out.
Question - do you, and your buddy, use your drysuit only for buoyancy control, or your BC?
If either/both of you have different methodology here then it can only lead to confusion where buoyancy control is most vital - on the ascent.
Consider an agreed approach to this, in the light of recent dives, and find a method that allows ease of management.
Others have posted correctly on the freeflow issue. You will only have colder teeth should you continue to breathe from a freeflowing reg. The surplus gas can go out the side of your mouth.
Your diving future?
Please do not go down the route of "Gear for Fear" i.e. believing that the purchase of additional tanks/regs/etc. will alleviate any future worries and forego any problems.
Instead, think of ensuring that your current gear is up to scratch. Reg? If not - replace it. Keep the old one for the pool.
As to the psychological needs, I truly believe - following your recent experience - that both of you are are a much safer and, indeed, a more practically educated diving pair than heretofore.
Go out and re-live that incident with the preparedness that only raw experience can give!!
Safe Diving
Seadeuce
