Cool, I'll have a play around with that later. Does the invisible sprite also need to have the main movement routines (eg I'm currently using MoveTo + Line of Sight to chase the player when they can see him), or can you assign those to the main sprite, and just leave the pinned invisible ones as Custom Movement with Solid enabled and it'll work itself out?
My main problem in the past has been the way sprites sort of 'pop' around the place when pushing out of solids, especially when clumped - it just looks glitchy and bad. A lot of my game is also in very tight tunnels which makes it doubly hard to find something that works.
No, use pin behavior and pin barrier box on start of layout to enemy.
Try widening your collision polygon. Open sprite edit and bottom left is lots of dots, drag them out to widest points, try to make circle round sprite this is the barrier for solid. See how that looks before adding invisible sprite, make sprite image bigger (the clear space) and widen the circle till it looks about right... see if that works. I say circle because square will give that bumping look when colliding. circle rolls around nicely.
Edit: monitz87 answered above perfectly, but try and play around with collision polygon first, see how it looks