When I use the drop down menu, and go to click on the area at the side, for instance a shop or forum post, it vanishes, so I can't use get to it from there.

Same here (unless I move the mouse extra quickly)

Also, the profile comments "reply" box doesn't appear (even when I remove all my customCSS) - the page goes black, but no box (unless I use the p2p page) :(

I'm not fast enough lol, it just goes 'zip' and disappears

My submenu now has a little scrollbar, it's hilarious.

yep, once the tiny font & other changes reverted back to normal, the drop down menu worked properly again :D