Any question answered...

If the rear brake lever is on the left hand side, why does the shifter for rear gears on the right hand side?

Euro style is left hand front brake & front derailleur, right hand rear brake & rear derailleur. Makes sense to me. Blame Brexit! Makes the front brake cabling neater as well (on side pull rim brake bikes).

Only argument 'against' it I can think of is that on motorcycles the right lever is the front brake. But I don't ride motorcycles so that's no problem for me.

Jan Heine has this to say about it:

Early bikes had only a rear brake. In France, this usually was a rim brake. The early brakes were not very powerful, so you needed lots of hand power to stop the bike. Most people are right-handed, and it made sense to control the single brake with the right hand. When front brakes were added to bikes sometime in the 20th century, this required adding a brake lever to the handlebars. In France, the right side was taken, so they mounted the extra lever on the left.