Thank you, I think it makes sense now.
So, something along the lines of Londina > Londna > Londra?
Yes, but it would have been Londinia (accusative plural) > LondniaI guess that it came to Spanish and Portuguese via French, judging by the final -es which is beyond usual among French placenames, as well as by geographic and historical ties.
Precisely because of that. There are many toponyms in Occitania which didn't have an s originally but it was added in the French adaptations: Tarbes, Lourdes, Arles, Antibes, etc.All final -s have been silent since the thirteenth century so adding -s would not have made any effect on the pronunciation.