Losing my primary title

Solution 1:

All that will happen is that your current King will take on a new title, your vassals will then switch to whichever Kingdom you have now become - regardless of whether England successfully invade Wales or not. Those vassals in 'Brittany' and 'Scotland' are YOUR vassals not your 'titles' vassals.

You do however have an issue if you lose your second kingdom as you will be forced to become a 'Duke' which means any vassals of yours (assuming you have Duke vassals) will no longer be of lower rank and therefore you will no longer be their liege lord.

Solution 2:

I believe that all of the territories that are considered to be part of Wales will become theirs. And this means not just de-jure Wales, but everything that is part of it.

An idea for a solution would be to set Ireland to primary and make some single-county guy King of Wales without all the other territory. Basically drop the king title for Wales, de-facto appending all those territories to Ireland and insuring that you come out ahead.

That's assuming you're about to lose.

To see what is considered part of Wales click on the title and see what is glowing in the non-de-jure mode.

Also, as the king, you'll get a strong claim on the kingdom in the event of a loss.