Two random questions.
1. If you shoot video in 30 or 60 fps will there be any problems or quality loss if you decide to convert to 24 fps later on? Why not just always shoot in the higher frame rate so you can use either later?
There won't be quality loss
per se, as the conversion will simply drop some frames. Since you've filmed more frames than you actually needed, you shouldn't see much of a drop-off in quality, though there can be a bit of jerkiness if there is fast action (because of the low frame rate of 24 fps) but otherwise, you're just deleting frames (2:1 if you go from 60 fps to 30 fps). Going from 60 fps to 30 is usually quite smooth. 60 to 24 requires a 40% conversion and most software is pretty good at this nowadays. Going backwards (30 fps to 60 fps) will require duplication and interpretation of frames and that's not desirable.
The reason you don't always shoot at 60 fps is that you can't get the higher pixel count (SD vs. HD vs. UHD, as a way to look at it) with the higher fps count. The Mavic, for example, is only capable of shooting 60 fps at 2.7K resolution. If you want 4K, you'll have to film in 30 fps. Additionally, many people don't want to mess with converting frame rates and just simply want to view their clips. Simple video editing also makes frame rate conversion a royal pain. If you're big into post-processing, then it's not much of a big deal. It's all related to your skill set and how comfortable you are in more complicated formats and editing.
2. What kind of quality difference is there between JPEG and DNG photos with the Mavic? How major is it?
There is a major difference between JPG and DNG, and it's not specific to the Mavic. JPG is a compressed (and necessarily reduced-quality) image and a RAW photo hasn't been touched. For lots of people and many functions (photos on the web, for example), that quality drop is not noticeable. For other uses and media (like high quality prints or viewing on high resolution panels), the image drop may be significant and quite noticeable. RAW also gives you all the basic information/detail that the photo was shot in, allowing you to make post-production changes that you couldn't manage in the compressed version. It's all a personal choice.
Here's a pretty good article outlining the real-world differences between RAW and JPG images. The Mavic also doesn't have the best quality processor to process and compress the image into JPG, so the overall effects are even more pronounced.
RAW vs. JPEG | The Ultimate Visual Guide - SLR Lounge
