I would use them. Experiment. Thaw a few and use them in some muffins and leave a few frozen and use them in the other moffins and see if there is a need to adjust the bake time. I would use them frozen.
I would at best guess say something was not sterile or the culture was tainted. Homemade products don't have additives to keep them for a long shelf life so what you make should be used in about a week.
If it is set up it is ready. There is no perfect time period for it. If you want it more 'soured' then let it go longer. Never let it go more than 24 hours as you could get some bad bacteria going in it.
You can shake it in a jar or use your mixer like you are making whipped cream and when it reaches whipped cream don't stop. Let the cream break and you will start to see little flecks of butter. Keep going you will succeed. :)
We stood outside this morning with my 2 yr old DS and watched a large formation of canadian geese fly over. They were loud. The honking set off my own geese.
It doesn't take as much as you think. 1 qt sour cream, 1 qt cream, 1 quart yogurt, 1 quart cottage cheese, 2 lbs butter, 1/2 gal buttermilk, 2 - 3 lbs cheese - etc - no more than a 3 - 4 gallons of milk a week and those products will last 2 - 3 weeks after you make them. You just have to expend...
If I lived in Wisconsin and had a dairy farm ... holy smokes the things I could do.
You can have more dairy items fresh and on hand than you could ever afford to buy.
Mozzarella, quesco fresco, queso blanco, buttermilk cheese, sour cream, buttermilk, butter, yogurt, cream, cottage cheese...
When you make simple cheeses that do not require aging or drying you can get more than enough cheese each week from 2 - 4 gallons of milk and still be able to process them again for ricotta and then use the whey for bread baking or giving to the chickens and pigs as good nutrition!