Atomic Perception

You can dial in some of that by distributing mass across the gradient of the mesh. Calculate a 0-1 index from the relative bounding box of the source, in this case the x-axis. Use that index to drive a ramp where you can remap and rescale the final value. The net result is a little bit of falloff, and not a complete collapse.

One thing to try is supplying an alternate collision object. Instead of placing your FLIP source inside the collision object and inverting the SDF, try building walls around the source and running non-inverted. When I turn off Oversampling At Boundaries on the Reseeding tab, I notice the white velocity spikes go away. My guess is when old particles die in a voxel, and are reseeded, the new particles might intersect momentarily, generating a velocity boost that reveals itself as those white spikes.

There's a couple of ways to approach that. One is to set some value on the MTLXStandardSurface node that you want to persist, then right-click and choose Make Permanent Default. Then any new creations of the MTLXStandardSurface will contain your edits.

If you drop down a USD MTLXSubnet and dive inside, you'll discover the barebones version of a surface that you can edit for your needs.
In LOPs there is also the editmaterialproperties node to non-destructively modify parameters.