Assuming I have more trade routes available to me that I haven't used yet, what determines the success of actually "establishing" a trade route. like:

is it:
1) 100% random with 0 factors other than that I must have routes available?
or
2) do things influence the success of establishing a trade route? and if so what

I know about the rare material I can find to get clans to approach me with trade routes, but when I click "establish trade route" in the trade screen I wonder if 1) do skills have a role? 2) how the clan views mine? because I have trade routes with people im near to feuding with at the start of the game 3) does any sacred time magic like trade or diplomancy influence it

best as I can tell, what it boils down to is how favorable the clan is towards me. but i'm still not 100% sure of that

So one way to approach this is to ask: what should matter? Chances are, it does. Clan relationships are modifiers to most skill tests in the game. And yes, the skill of your negotiator matters, like you would expect. (You would not send a non-trader to set up a trade route.)

Starting trade routes were presumably in place long before relations soured.