I've been using MPEG-Streamclip and have had no problems and still seems to me one of the best free options out there. What kind of problems are you facing?

Update: I've been searching for options to let you know but still, MPEG-Streamclip seems to be the best free converter to DNxHD. Many of the free ones suffer the same as Handbrake.

You will have to record it all backwards. The person will have to "lip" the sentences backwards. When you reverse the footage it will come out right.

Try to make some short films first. Just to get check what it is to be in a shooting and editing position. If you dont have ideas for a short, shoot a scene or scenes from the movie you want to make a feature. No need to release it to the public, keep em as tests. Just shoot, edit it and keep it between your friends.
And yes, it is possible to shoot a feature with a little crew of friends. Read "Rebel Without a Crew" from Robert Rodrigues and get inspired. If he made it back then, we all can make it now since everything we need is much easier to get.

BMPCC as a low budget camera it is quite nice. You got raw video and such. But I'd get a a6500 or a g80/85 instead. The BMPC has terrible battery life and it has a bit of a problem with IR. I would get a C100 anytime over any of those the if 4k is not a must.

There are no 4K budget Canon cameras. Look for Sony a6300, a6500 or Panasonic g7 or g80/g85 and get an adapter. Take a look online and see which will suit better your needs. This assuming your buget is under $1500.

I would use denoiser as you stated and then sharpen the image back on the zones I want. Mask the subjects, bring a bit of light back and sharpen it until you like it. As for the out of focus, the same apply but it may not be perfect.