The usages of "would" are almost endless. Even though one definition of the word is a past tense of "will", in this case it is more like a prediction, almost "future tense". Read Merriam Webster's Definition
"You know Bruce would loan you the money, just ASK for it!"
"Bruce is running for Governor. He would loan you the money if you could get that bill passed."
As to your second question, absolutely. Each of those words could be substituted for "would" and the sentence would have a completely different meaning.
"Could" would say that he has the money, therefore he can lend it to you, but it gives no hint as to whether Bruce would be inclined to do so.
"Will" says that Bruce has the money and will be happy to loan it to you. That Bruce will make the loan is assured.
"Might" is a big question mark. Hmmm, it's worth a shot.