I like this idea! As it would take quite a while to melt its way down, you'd basically have to sink the entire rover or a submodule down with the radioactive heat source (not melting a hole then exploring it). If it's a one-way, single-purpose trip then it's not clear to me that the ice would necessarily pose any sort of issue with non-line-of-sight (aka radio) communications.

Water in and of itself isn't a great communications barrier, but the conductivity of sea/salt water causes it to act like a Faraday cage and kills/significantly dampens any radio waves attempting to pass through it.

Under the assumption that Europa's oceans are in fact salty, the frozen sea ice isn't actually all that salty. Ice in general isn't a good conductor as it's actually almost entirely the pure water that freezes leaving the salt behind (I'm not clear on whether it would precipitate but remain suspended between the the H2O molecules or if - given the lower density of Ice I - the water would freeze from the top down leaving the liquid seawater beneath it ever so slightly saltier while forming a layer of pure frozen ice on the top).

In all cases, keeping a base station while sinking a second module with the radioactive heat source then proxying all communication via the base station would be possible even if the ice did form some form of a Faraday cage that severely limited communication from beneath.