What do you mean they don't support Bootstrap 4? They shouldn't need to unless you mean the colors don't match. Responsiveness in CB and our plugins was also significantly improved with this release so should display perfectly fine on mobile, desktop, etc..

Is it possible to have community builder 2.2 with the old bootstrap?

No, the entire point of CB 2.2 is Bootstrap 4.

Pictures in CB Activity: I had made CSS so that the picture was stacked above the link (both were taking up 100% of the width) and metadata inside any Activity. Now they are side by side and it looks quite a bit less attractive as my users love big pictures. How can I stack them above one another again?

Pictures displayed by CB Gallery on your activity stream? Are you using a custom template in CB Gallery to modify its activity display? If so change back to default as you'll need to update your template. It's safe to assume you'll have to redo any custom CSS you're applying to CB.

2: CB Activity: The image vanishes when I look at the website over the phone. Anything I can do against that as my users love looking at photos on the phone.

Not sure I understand. Images are displaying perfectly fine in my tests when viewing mobile view of a page. Are all images being hidden? Those from CB Gallery and those linked to using attachments? Try switching to default Joomla protostar template and see if issue resolves as you may have a CSS conflict coming from somewhere.

3: For some weird reason, I can only change the size of the LIKE button + writing IF I change the size of the authors name as well (both are set at 0.89rem and ".textsmall" is set at 0.76rem). I would like the LIKE button to be the size of text small.

There's a element specific CSS class for both. For the like button you'd style likeStreamLink class and for the names you'd style likeStreamCount. Use !important flags to force your styling where necessary. So if you want the button to have same size as text-small utility class you'd use the below for example.