This website is hardly the best on the subject, but I EASILY found information useful enough to make your statement meaningless concerning Casse.
http://dnghu.org/indoeuropean/indo-european.htm
"NOTE. Later evolution of Celtic languages: ē >/ī/; Thematic genitive *ōd/*ī; Aspirated Voiced > Voiced; Specialized Passive in -r.
Italo-Celtic refers to the hypothesis that Italic and Celtic dialects are descended from a common ancestor, Proto-Italo-Celtic, at a stage post-dating Proto-Indo-European. Since both Proto-Celtic and Proto-Italic date to the early Iron Age (say, the centuries on either side of 1000 BC), a probable time frame for the assumed period of language contact would be the late Bronze Age, the early to mid 2nd millennium BC. Such grouping is supported among others by Meillet (1890), and Kortlandt (2007).
One argument for Italo-Celtic was the thematic Genitive in i (dominus, domini). Both in Italic (Popliosio Valesiosio, Lapis Satricanus) and in Celtic (Lepontic, Celtiberian -o), however, traces of the -osyo Genitive of Proto-Indo-European have been discovered, so that the spread of the i-Genitive could have occurred in the two groups independently, or by areal diffusion. The community of -ī in Italic and Celtic may be then attributable to early contact, rather than to an original unity. The i-Genitive has been compared to the so-called Cvi formation in Sanskrit, but that too is probably a comparatively late development. The phenomenon is probably related to the Indo-European feminine long i stems and the Luwian i-mutation.
Elmetiacos, did you even do research into true Celtic linguistics beyond internet fandom? What is your method for reconstruction from IE or are you seriously trying to say that Late Celtic language resembles the language of 272BC? I could be wrong, suredly, since I have barely spent time on it, but already I have found information contradicting your own. Is there some rule I should know of that has been dated to have taken place concerning e > i or IE genitive or some other Celtic language rule in effect? If not, I really wonder why you seem so certain.
Also, you assume that Casse is supposing to be nominative plural, but how do you know?
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Actually I have found some evidence in some Germanic loanwords from Celtic - 'wire' which can prove that the monopthongised ei to ē being different between various forms of Celtic, nonetheless important in their date during EB timeline, unlike Elmetiacos' assumption that all Celtic was fully transformed:
"These forms reflect an IE source ueiros meaning 'curved, twisted' but presuppose transmission through Celtic in its varying dialectal forms. Insular Celtic monopthongised ei to ē (cf. Old Ir. fíar, We. gwyr 'crooked') whilst on the continent the same development to ē took place (reflected in Germanic ē2, hence OHG wiara), but also a different monopthongisation to ī (hence OE wīr). To assume direct descent from IE would not account for the variants found in Germanic, which instead reflect dialectal differentiation in Celtic. When this loan is to be dated is more uncertain" (DH Green 155).
Elmetiacos, it is true that a nominative plural makes most sense, but you also do not know why an -e is there. as you say, a locative or whatever is silly... so there is more going on than a simple gloss. and Casse are not Gauls either.