They blow up randomly as far as I know. If you try that potion again, it won't necessarily blow up, but it may blow up a number of times in a row, or it may work perfectly fine.

To eliminate the risk of potions blowing up, you will want your alchemy up to level 3

Explosions are totally, completely random. There is no potion, not a single one, that requires alchemy. I've finished every puzzle in the game, and the alchemy challenge, with Alchemy 1.

Potion 112, 212, and 211 are all the same potion, and all three together count only once towards the 52-potion total.

do the villagers have to use the potion?.. or can i just make the potion without having anyone taste it? to complted the 50 recipes?

Wrong.

212 has 2 twos and 1 one

112 and 211 have 2 ones and 1 two

No. So long as it is completed successfully without blowing up, it will count as one of your 50.