I've watched lots of videos on how to prepare bone broths. However, I haven't found much information about making it directly from frozen bones. I have understood that I should move frozen bones from the freezer to the fridge at least 1 day or 2 days before to be safe. (If I want to use them normal way) Or that I should put those frozen bones in cold water for at least 1 hour before use. But if I want to use them directly from frozen? Can I? (without roasting them, directly just like that) If I use a Pressure Cooker, on Low or High Pressure, I don't think there is any problem. But if I want to use a Slow Cooker? Generally, for frozen meat, they advice to set on High (Slow Cooker) for 1 hour and then move to Low. But is it safe to do the same with frozen bones? From a bacterial point of view? Or should I boil them in a stove pot before (for how long?) and then transfer them to the Slow Cooker? Maybe I'm overthinking it too much but I prefer to understand the process if possible. Thanks for your help. 
The key concept is the food safety 'danger zone'. There is no reason to treat bones (which will have traces of meat on them) differently to meat. Whatever temperature they start at, you want to avoid the bones spending too much time in the danger zone temperatures. Starting from fridge temperatures rather than freezer temperatures helps this because your stove or cooker will be able to bring them to a cooking temperature faster. As long as the quantity of bones isn't huge, starting from frozen in a pressure cooker or on a stovetop should have no problem getting to a cooking temperature quickly, but a slow cooker on its low setting might be an issue, as you have noted – follow the advice for frozen meat of using the high setting initially. 