The first is that I need to implement some culling of objects that need not be considered to render the shadow maps of (I haven't really looked into this yet and it is probably simple enough; I'd imagine I can just perform my usual frustum culling routine using a ortho view-projection matrix that has its near plane pulled back to the light source (or rather the length of the scene camera's z-range) but is otherwise the same as the light matrix of each cascade split?).
Yes, you can perform standard frustum/object intersection tests in order to cull objects for each cascade. Since the projection is orthographic, you can also treat the frustum as an OBB and test for intersection against that. Just be aware that if you use pancaking, then you have to treat the frustum as if it extended infinitely towards the light source. If you're going to cull by testing against the 6 planes of the frustum, then you can simply skip testing the near clip plane.
The second is how to get this shadow map interpolation working properly. I just whipped the following up for testing, it doesn't really create any visible difference from just leaving the interpolation part out alltogether, but am I going about this in the right way or would I be better off to change my approach?
Generally you want to determine if your pixel is at the "edge" of a cascade, using whichever method you use for partitioning the viewable area into your multiple cascades. You can have a look at my code for an example, if you'd like. In that sample app, the cascade is chosen using the view-space Z value (depth) of the pixel. It basically checks how far into the cascade the pixel is, and if it's in the last 10% of the depth range it starts to blend in the next cascade.