Chicken and egg question. So the て-form of だ/です is で. But the で in だ/です comes from the particle で. So are these でs considered identical?
They’re completely unrelated as far as I can tell.
The で particle indicates the context or means through which something takes place. だ is the “is” in an “A is B” type sentence.
で in its linking form as the te-form of nouns to give reasons is derived from the copula as far as I can tell not from the particle.
Whoops sorry I misread your post!
Ok, follow up: Is the で in である the particle で (or related to it) or is it its own thing?
である is a copular verb, so no it’s not derived from that particle で.
I think the で in である is the “て-form” for だ (as you put it). More accurately, で is the 連用形 form of だ, and then you attach ある to it. I think that’s what’s happening from a etymological perspective. Honestly, て-form is one of the few things I’m having trouble finding sources on from a native grammar perspective.
EDIT: Well never mind.
Wiktionary also more specifically says its etymology is from a contraction of にてあり
Now that’s just weird.
You beat me to it!
If you look up the particle で it says it’s “originally a contraction of にて, later treated as a conjugation of the copula だ.”
Well then I stand corrected. Interesting info I should have looked up. 
Always love learning new things like this. There’s always so many new interesting nuggets to learn about Japanese.