The words "for" and "if" may often be found next to each other, but that does not mean that they represent a coherent structure.
In each of these examples we find a fronted adverbial clause introduced by "if". We also find "for", which introduces the entire matrix clause including its subordinate modifier. The "for" and "if" are next to each other only because the adverbial clause appears before the main clause. We could place the adverbials in their canonical locations, which would naturally separate the "for" and the "if":
- For might not this radiance reach the eyes of Silver himself if I could see it.
- For I tell you we must die if we don't find the next little girl.
- For we should have perished if it had not been for that.
- The original motivation is convenience, for c disappears from our equation if we designate it as unit velocity.
The reason that you can't find a meaning for that pair of words is simply that those words are not paired.