Did Maxwell ever mention the scientific concept of "sloshing"?

No. The electron had not yet been discovered.

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Electrons sloshing about in conductors, in the same general direction,
always attempt to avoid each other.

This unsociable characteristic results in a pressure which drives them
to flow near to the surface of conductors in which they are sloshing.

Hence skin and proximity effects.

There is an opposite effect. When electrons slosh about in opposite
general directions they form a great liking for each other.

The result is a mechanical attractive force between a pair of parallel
conductors carrying current in opposite directions. Also another
proximity effect.

It's all so simple. Can't imagine why you have sloshing problems. But
no doubt Cecil will introduce reflections, standing waves on meters
which don't measure them, and SHF scattering parameters. ;o)
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Reg, G4FGQ