Satella's name is not a combination of Subaru and Emilia's names? I don't know how you reached that conclusion. In neither Japanese nor English is her name close to bring a combined form of theirs.

Her name is likely a corrupted version of the word "satellite", as in a satellite such as the moon. The Warlock of Envy in Mimagau IF is named Luna, (Natsuki) Subaru has "moon" in his name, and something with Emilia was recently revealed to be derived from the moon, can't remember the exact wording on that last one. But the point is that Satella's name being a reference to the word "satellite" fits the whole moon thing Subaru and Emilia have going on.

I don't think too overly complicated for this. I remembered in Rave by Hiro Mashima, the FMC which is ELIE is a historical figure who get sent into the future and forgot all her memory but the number of 3173 in her arm, which is being read as ELIE. It's fun but impactful. So,

SA+ELA = SubAru + EmiLiA

That's a stretch. It's effectively the same thing as saying you could get the name "Crusch" from "Schult" and "Camberley". It doesn't mean anything. The names don't even fit as a "combination" in English, not to mention the kana of their Japanese names doesn't fit together. In order for your conclusion to be plausible, the kana of their names would have to match up, but they don't. The author wouldn't hide lore in English transliterations of character names that would not have even gotten published had Re:Zero not been adapted into a light novel when there are much more reasonable conclusions that a reader could draw.

The fact that the Warlock of Envy in one of the alternate universe IF stories is named "Luna" is basically screaming that Satella's name likely has some relation to the moon and is another astronomical reference in general, as is typical of this series.