I encountered the "Secret Purgatory Ending" and decided to do some testing in case this proves useful. In all versions I have the 2 keys and the fishing pole as well as 3 milk picked up.
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Drinking 0 Milk sends me to purgatory
Drinking 1 Milk before entering the saucer and refusing to drink the other 2 allows me to get the ending for Unstable formula
Drinking 1 Milk before entering the saucer and attempting to drink more gives you a dialogue box saying X2 plans to save it. (Pretty sure this is intended)
Drinking 2 Milk before entering the saucer and refusing to drink the other one... clones X2 when you use the milker for some reason? The X2 clone has a breast level of 0. You get Unsatable Formula ending
Drinking 2 Milk before entering the saucer and attempting to drink more gives you a dialogue box saying X2 plans to save it. (Pretty sure this is intended)
Drinking 3 Milk before entering the saucer and going as normal results in no issues at the milking machine, and the same (minor) issue at the synthesiser I reported before where X2 says she needs to milk herself first but then goes ahead and uses the machine anyway (you get the moderate formula)
Also just for more data points these are the items I have on me before I drink any milk
