Hello. I find this topic very interesting because we have (we had, at least, for what I remember) the same question in breton. I don't know now what has been decided, but I remermber that during a certain time the solution was to indicate the mutation and let the "native" form at the same time, using a slash
For example : I'm going to Kemper (or "to Kemper I am going" (depends on which element you want to insist) "to Kemper" would be writen "da G/Kemper" (or maybe the contrary, da K/Gemper, but it semms to me that the mutated letter comes first. Erwann would say if he comes on this topic)
That's for the writing. To be said, it would be with the mutated form. But I think as Robbruce : some mutated forms come "alone", you don't even think, and others would not, they would sound "bizarre" with their first letter mutated, and you'll prefer tell their name in the usual form you are used to
For example "da Zublin" sounds me better than "da Dublin", but "da Vadrid" sounds me rather "bizarre", I would say "da Madrid", and that's why, I suppose, there is (was ?) this writing with the slash...
But for the countries it is more simple, because they generally begin with "Bro" (land) "Bro Iwerzhon" = Ireland, "Bro Gembre" = Wales, "Bro C'hall" = France) which makes a mutation Well : I should say "began", because I can see that it is now changing : a lot of new speakers do not use "bro" anymore. They will say "Korea" "Iwerzhon", (and they even say "Frañs", which is in fact the phonic french form for France, instead of breton Bro Ch'all)
I like the using of the slash, because it can be not so obvious, for a new speaker, to recognize some names when they mutate (Zublin.. Zeppelin ?...)
You see, That's why I find vrey interesting this question of Sororp (not at all "a stupid little question") and I read with interest your opinions