Stpug, what volume are you freezing and what thickness of slurry? I'm considering using 250ml canning jars. I would mix 70ml water with 30ml glycerine per jar and pressure cook them 15min at pressure. I could let them cool while staying sterile that way. Then I plant to pitch one wyeast pack to 1L oxygenated wort and let that ferment out. When all done I'd swirl up the starter so its homogeneous and add 100ml to each water/glycerine jar, close lid finger tight and freeze. Once frozen I'd tighten the lids. This figures to 10 frozen cultures in 15% glycerine. Id store them in the freezer inside a small, thick walled Styrofoam cooler I have with a couple gell packs, to protect against the defrost cycle.
Does that sound right?
I've heard two thaw methods. Quick in 100F bath and pitch quickly. Or slow thaw two days in fridge then 1 day at room temp. I know you've said you use the quick method. Have you tried the other? If so what were results?
I also figured I would do a two step revival starter, probably 500ml till fully fermented, decant and then do my normal 1L oxygenated high krausen pitch. Sound about right?
I freeze in 50ml centrifuge tubes with collar (collar helps them stand). My process is fairly simple. Prepare a mix of 50/50 glycerin/water and sterilize to your personal requirements, and chill. Centrifuge tube is filled with ~31ml yeast slurry (fairly thick; assumed 90-100billion cells) and 13.5ml glycerin solution to produce a WhiteLabs-equivalent vial of yeast at ~15% glycerin content and enough head space for freeze expansion. Agitate enough to ensure good mixing. Directly into deep freeze. I should also mention (because I had the same question back when I started), the frozen vials freeze solid - they do not remain liquid.
I chose the 50ml centrifuge tubes because, when refrigerating yeast under beer I quickly found out how much space it can take. Once I got into freezing the yeast I wanted to be able to keep several strains on hand for future use. Quite honestly, if I were to do it again I might opt for smaller 30ml tubes and just plan two-step starters for each use just to save even more space. The 2-step starter process you have planned is a good way to check for any problems with your frozen strain since you can judge performance and fermentation characteristics during that first step, and then decide if you should continue with that frozen culture or not.
If your freezer has thaw cycles then your styrofoam cooler is helpful. Folks will often times include containers of isopropyl alcohol in the foam cooler to help stabilize the temperature during the thaw cycle.
I've used both methods of thawing yeast: Quick and slow (2 day fridge thaw). I did not see much (or any really) visible difference in how this affected the yeast propagation/lag/etc. Since I saw no difference, and recognize that the yeast begin using their reserves once they thaw, I opted for the quick method. The other benefit is no prior planning two days before making a starter.
As far as "freezing not being proper", while not the most proper way of preserving yeast for future use, I can definitely say that it works and works well. I wouldn't continue to do it if it wasn't reliable.