@happy-diver
Lots of snorkeling practice with mask or goggles and snorkel.
If I felt that mouth breathing was interfering with keeping water out of my nose, and for some reason I couldn't expel it, I closed my nose with my fingers and this allowed me to easily regain control of my breathing and I immediately calmed down.
The main problem was emptying the mask, basically even if I blow from my nose, the water finds a way to get in, to the point of making me start coughing. If that happens, I again partially close my nose with my fingers and try again, keeping my nose partially closed, this way I always solve the problem.
It may not be the most elegant way to empty the mask, but I don't care, this allows me to do all the dives I want in peace, both with scuba, accompanied or not [I've a couple a tanks] and freediving.
And I'm calm, because I've verified several times, in my tests (for example replacing a mask underwater) that now I can handle this kind of situations, even if not exactly the way I was taught....
Basically, the way I see it, you have to be able to handle the water in your nose and you have to be able to empty the mask, and always with what you have available.
Well, having said that, there is also the issue of comfort, you have to be able to handle the water in your nose and so on, but it is not a given that one has to endure it on every dive...after all I dive for pleasure.