Weight is my go to for any measurement I want to be reasonably precise, solids or liquids. my kitchen scale gets a lot of use.

that said, if this is a baked good like a cookie or brownie where you enjoy biting into the existing texture, I'd probably go with less precise and just cutting it by eye in whatever way seems less destructive. 1/5ths is a little tough though. I'd tend to go for 1/4 or 1/6 and feel better about the precision to cut something in half and then half again or in thirds and then half again.

I don't think It's possible. You can't know that everything is distributed evenly in the cookie. So even if all pieces are the same size, you may have a piece with more in it then the others. This is called a hotspot.

Don't know if it will be like that but it is the first thing that comes to mind.

I'd personally just cut it up into even looking squares and call it good. I'm not really going to care if I get 18mg in one brownie and 22mg in another. Crumbling it all up kind of ruins the appeal of having a brownie to begin with.

Ya weight is how I'd do it, it'll probably be the most accurate. Realistically you don't need to be accurate at all but with something that doesn't cut cleanly like a gummy might, just weigh it

Theres always going to be some imprecision with baked goods. Just try your best to cut it apart equally and hope it all averages out. If you want to get scientific and exact you should put the cannaoil in pill capsules.