The Definitive Rules of Freezing - please educate me!

[WopBopALooBop]

I am going to make a chicken and mushroom pie this avo, with a left over roast chicken which was roasted a few days ago. Will it be safe to then freeze the pie, given that the chicken will have been cooked twice? Some people think this is dangerous but I don't understand how it can be if it's cooked meat. Surely now it's cooked it will stay cooked and there won't be any harmful uncooked-chicken bacteria once I defrost the frozen pie?

Clearly, I have no idea what I'm doing.

Could someone please give me the low-down on rules for freezing and de-frosting and re-freezing? Is re-freezing ever ok?? E.g how can berries that have been frozen and de-frosted then be harmful if re-frozen??

Sausages/burgers bought from shop - freeze on day of purchase, and then don't re-freeze if you defrost them? Yes? But why?? You can freeze cooked sausages I guess, like you can freeze cooked mince in a lasagne, for example?

Ahhhh I would be so grateful if someone could clear this up for me haha! :)

This is a good web page:
here

With your pie, you shouldn't need to cook it, just put the roast chicken in white sauce into pie crust and freeze it straight away. When you want to eat it, thaw overnight in fridge and cook straight away (or cook from frozen and give it more time) either way, check the middle is piping hot. These are my type of guidelines and we have never been ill

The thing about refreezing raw meat:
Fresh meat has tiny amounts of potentially harmful bacteria in it. Freezing it halts bacterial growth. Thawing it warms the bacteria up so that can start to multiply again (like if you had kept the fresh meat in your fridge for a week). Cooking after thawing will kill the bacteria. Refreezing after thawing just halts the bacteria at their new higher level. Thawing for the second time causes more bacteria growth, and so on. Some food is nicest when only lightly cooked (eg thin pork escalopes), this level of cooking might not destroy the increased bacteria and might cause illness. So it's all about time and temperature really. But having a policy of never freezing something twice in the same state (eg raw/cooked) is a good idea in my mind.

Ahh thank you, this is all so helpful!! :)