Sorry to say, you will have to buy full frame cameras from Nikon or Canon which cost an arm and a leg. But if I were you and money is an option, get the Panaonix DMC-FZ1000 or the G7 Lumix. Both film at 4k and are not bad in filming in the dark. You will be surprised with the quality.

Mirrorless or (Micro 4/3) cameras takes photos without a mirror. Meaning there is NO pentaprism. The catch in having mirrorless is once you look at the electronic viewfinder, what you see is what you get. ON a DSLR, you look at the viewfinder and what you see is sometimes something you do not want to get. So if I took a picture on a DSLR but my viewfinder shows it OK, chances are it will come out bad unless you are a pro and no manual photography (not automatic).

Yes it can, you buy an adapter. So you can buy a Lumix G7 and buy an adapter to fit Nikon lenses or Canon lenses. The catch is what kind of adapter you buy. You buy a cheap adapter, the metal fillings peel off whereas if you get a speedbooster adapter, the quality is better. But if you buy an adapter, you will no longer autofocus, you will have to do it manually which is better believe it or not. If you have questions, let me know Jeff.

I would get the Panasonic Lumix G series or GH series. You can record 4k were the Olympus cannot.