When you create a Variant, all your content is copied to a new page the size you have specified. The content is linked to the main variant.
Yes I agree.
Frequently in adjusting the size or position of something in the mobile variant, I find the same object gets moved or modified in the main variant. If I change the size of the text in the mobile variant, even if I create a new Text Style, some of the text on the main variant is changed.
The main two assests are either text or images.
I've never had issues with resizing and moving images within variants, so I can't understand why you are? Strange.
Simlarly, selecting some of the Normal Text Style and amending it, such that the style gets '+' sign next to it and then selecting "Update Normal Text in this Variant" does exactly as it should.
It is not consistent and it is not predictable
That's very odd, I've never had issues with variants. Perhaps it's a system set-up issue.