This is the shittiest test footage I've seen in a long time. I've seen children film better. Anyway, the inbody stabilisation is really not mendatory, and in this case it wouldnt have been powerfull enough to save the shakiness of those hand. It's been only a few years that sensor are stabilized on camera that are used for video and we did film handheld before, just try to hold still and use a stabilized lense and it will be fine.

I saw people using that kind of setup on press event, I tried it myself but I found the idea not as efficient as expected as you have to adjust the angle of the top camera all the time as a nice position for a close up doesn't especially give a good wide shot. It can be usefull for an interview setup but I wouldn't advise it for run and gun

If you want to keep using your canon L lenses then the A7sII makes more sense.

In therms of all in one camera I like the sony X70, great battery life (8hour), 4k, nice low light, extremely good stabilisation. It's the one I would probably use for a trip like this. If you want a camera that does both still and video the gh5/gh4 is probably the best choice.

Imovie only allow for a very basic edit but cannot provide a professional result. One exemple of how limited it is, is that you cannot do any audio crossfade between shot. If you aim to make a familly film it's fine, if you want to reach a larger circle you need something better.
I would advise to go for final cut pro X as it is easy and cheap and there is almost no limitation in what you can do as solo editor compare to other pro editing software.

For me the most interesting camera is the c200 as it has the best versatility, it can do great run and gun images with a reliable autofocus for small daily job and be also used to shoot raw on high end projects. I do like the fs5 too but the canon c200 is just better in every area, all I miss now is a good affordable 17-110 all in one lense for those little jobs.