I cannot seem to negotiate any kind of deal with factions who have higher power ratings (even ones I supposedly have good relations with).
I find that since playing on the harder difficulty gives the other factions such insane bonuses to resource production, the player is basically left unable to trade almost anything for any kind of value. You would think that they might actually add the weight of the resource loss for the player to the value of the trade. It makes a diplomatic victory on these difficulties almost impossible since you will ultimately be forced to initiate resource wars as the OP has suggested.
It is also very easy to prey on weaker factions using diplomacy
I think this point is simply an underscore of the fact that the relative loss/gain is never added to the value of the trade. I think it would nicely augment the diplomatic sphere if an AI added some value based upon the relative gain or lose of either side. Like, if I am super weathy and I try to cripple an opponent's faction by trading away their last stockpiles of a certain resource, then the AI should add a weight not simply for their large loss, but my very slight gain.
Other leaders kept initiating diplomatic conversations with me implying that I could trade for some tech they had discovered
This has been well discussed in other threads. It is generally agreed that these messages are a bit misleading as they are not connected with some diplomatic offer waiting for you. Most of the time it is just idle chatter with only limited substance.
since my wife died childless in the first 50 turns, I was for some reason destined to die a lonely bachelor.
Ever wonder why the sov is not an eligible target for an arranged marriage? I can understand the no new wife thing, but it always boggled my mind that my bachelor sov can not arrange a marriage between himself and another faction.