If you "send a diplomat home" it means you are declaring them "persona non grata". (sp ) you are actually banning them from re-entry. referred to as being PNG'd.

The problem is that often this would result in a tit for tat expulsion from the other country with one of our diplomats.

The whole point of diplomatic immunity is so that your diplomats can act without fear of arrest. While some countries might abuse it if it was not there then our people could be constantly being detained on spurious charges.

Yes but countries like that are not worth dealing with anyway.