Hey Leif,
If I may and speaking from past experience with this issue myself, it sounds like to me you are not really getting out on those outside edges. The sliding of your inside foot is very good indicator that your are trying to make the turns by turning that inside foot in the direction you want to go (causing the slide) instead of actually using the outside edge itself.
Now the question becomes, why is this happening?
Could it be the new stiffer boot, blades? Or is it your experience and comfort level in using those outside edges?
Again and this is only my opinion but I don't believe the 1/2 hollow has anything to do with it. I run Step Steel blacks with a 5/8 hollow on my Bauer 1X's and they have more than enough bite on any type of directional changes I employ providing I use the edges correctly. It really sounds like you just need to spend a bit more time working those outside edges and trusting yourself to use them. It can take a minute to really gain the confidence to lean onto those outside edges so keep at it.
To answer your other question, when I first started, I used a 7/16 which had a huge amount of bite but it really helped me develop the confidence to use my edges because they really dug into the ice and that made me feel more stable. I found myself confident enough to lean further and further out on the edges with each skate because of that stable feeling. As I got more comfortable, I began to back off and went from a 7/16 to 1/2 and now the 5/8 and it's great.
All the best.