I imagine someone here has taken the trouble to reconstruct the known Gothic names. I would be very interested in this. For example, Theoderic is actually Thiudareiks, meaning Ruler of the People. I suppose it won't always be possible to ascertain the originals and their meanings, but it always productive to give it thought. I am tired of seeing the names in Latin, English or Spanish.
Really interesting. There are just a handful of Germanic names with an initial sigis- theme (according to Förstemann 1900: Sigisbert, Sigisfrid, Sigismeres, Sigismund, Sigistricus, Sigisvulth) I can add a Sigesgundia in Galicia in 887), but there are plenty of them with just *segi-, and also with *ses/sis- (including Galician medieval names Sisulfus, Sisericus, Sisvaldus... and Sisuldus, Sisina, Sesinus, Sisilo, Sisbertus, Sesgundia, etc.) So I think that your reasoning can be also applyied to the identification of *sis- as a variant of *sigis-.
Now, on Rosamunda, etc, the element Maur- present in Maurila was probably taken from Latin Maurentius; Flor- in Floresindus from Florentius (flos ‘blossom’, and so 'to grow, prosper'); Cresc- in Crescemirus from Crescentius (crescere ‘to grow’); fortis in Fortesindus and Gundifortis is Latin fortis ‘strong’... So Rosamunda/Rosemudus can be related to Rosalia, Rosula or Rosina, but there are alternative Germanic etymologies (I concede that they are too many and probably too weak).