Yes, our bodycam is equipped with a 1/4" mount thread, so you can mount it on your bicycle bracket. 

Yes, no other accessories needed. The filter can be directly screwed on your camera lens. 

Yes, the video camera can record while it is charging. 

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EOS-FX,You can use Canon EOS EF / EFS Lens on a Fujifilm FX Mount Camera body,, Your canon ef 75-300 1:4-5.6 is canon EF mount lens and fuji x-a3 is X mount camera body. so the adapter ring fit for your camera and lens. but pls notice, your lens is telephoto lens,we do suggest you could use one telephoto lens bracket that could be attached to the tripod to increase its stability. because the canon lens could not adjust aperture,so suing our adapter ring could not adjust aperture