A super job!

IBIS works but you need version 2.5 of higher from Metabones.

Here is a clip using the Sigma 50-100 with the Metabones adapter at night, shot mostly at f/1.3

https://www.youtube.com/watch?v=0-E2-gf4Pc4

The alternative is to keep it and window box the video.

While upscaling (even by such a small amount) won't make any dramatic changes in normal footage it will be noticeable for titles and very sharp elements. If you have to upscale you would ideally want to superimpose titles after you do the upscaling.

It would help if you provide a link to your footage otherwise how can we help?

It disagree, buying a GH4 with the GH5 down the block is not good purchase. I expect that once the GH5 is out you can buy a GH4 below $500 due to the fact that almost every Panasonic GH4 lover will want to upgrade.

You heard wrong, YouTube and Vimeo respect the original fps.
And 24p is absolutely fine.