I think the main reason why the inscription was in elvish was because the dwarves only spoke their language among themselves and they didn't teach it to anybody, they preferred to learn other languages.
Daniel Telcontar, Elendur and Isildur aren't the same person. The name Elendur is quite common, Those that I can remember now:
Isildur's eldest son
The 9th King of Arnor
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