Oscar Wilde has a short story called "The Sphinx Without a Secret," about a woman who appears to have a great mystery in her life - going to a private apartment at certain times in the week, etc. After she dies, someone who's fascinated with her investigates, and finds out that the great secret is there was no secret - it was just a sham to create the appearance of mystery and excitement.
Donald Trump's Russian connection isn't quite the same as this - he isn't trying to create a mystery regarding whether he is a foreign agent. But the underlying truth here is, I think, as unmysterious as hers. I believe one can predict it with confidence, although obviously verifying (or modifying) it would be desirable.
Here's the basic model for understanding it. U.S. politicians often have big donors who give them lots of money, and with whom they have close relationships and shared understandings, while generally avoiding express quid pro quos. (Plus, each side in such a relationship does in fact have multiple allies, along with multiple parameters guiding its behavior.)
Not to pick on Republicans here relative to Democrats, but a classic example would be Scott Walker's relationship with the Koch brothers. They've given him lots of money, he tends to pursue their objectives, but he could honestly say that he'd do so anyway. He tends not to talk to them much directly (a liberal Boston radio jock embarrassed him by calling him on the phone and pretending to be a Koch; Walker fell for it and spoke with him very accommodatingly). Part of the donor's game is simply to exert influence over the donee's worldview, plus of course they have selected each other based on prior compatibility.
I would start with this as my model for Trump and the Russians, only there are some differences.
1) The Koch brothers are American citizens with strong views about U.S. policy. Russia is a foreign power whose leader regards the U.S., in large degree though not always, as an enemy. Putin recognizes that we have some common interests and foes, but his chief foreign policy goal is to restore Russian-U.S. parity or better, including Russian dominance in its territorial sphere at the expense of NATO allies. Hurting the U.S. is generally good from his standpoint, all else equal, and he would certainly like to expel us from as much of Europe as he can.
2) Scott Walker, I presume, has lots of funding sources. By contrast, I gather that there is substantial evidence that Trump has been heavily reliant on Russian financing for more than a decade. Other than Credit Suisse, I gather that few if any banks active in the U.S. credit market will fund him anymore, because he has proven to be too bad-faith a business partner.
3) Scott Walker knows and understands the norms of U.S. politics, e.g., regarding how you moderate the degree of quid pro quo. (And again, there are Democrats and their financial backers whom I could sub in here.) Trump, needless to say, doesn't understand this, and has zero inclination to try to learn it.
4) Scott Walker is within the psychologically normal range of human nature. Trump is an extreme narcissist. So the fact that these guys are his "friends," and have been helping him out and working with him for so many years, means that they and everything they want (unless directly at his expense) are by definition good.
I suspect that mostly covers it. I think that, if you asked Putin, under truth serum, is Trump your stooge, he would say Yes, and we've worked hard for many years to make him one. If you asked Trump under truth serum, he would say: No (obviously, this would be unacceptably hurtful to his self-esteem), but he would add that the Russians are really good people, they've helped me out, and I sympathize with them and understand their needs.
Is there an implicit threat that they'd pull the funding if he didn't give them what they want? I don't doubt that they would, but I presume he is not focused on that, and may even be naive about it. (Why would his friends, who are such nice guys, ever do that to him?) There's often an iron fist under the velvet glove of an ongoing relationship - but that doesn't mean that express blackmail threats are being regularly (or ever) issued.
Does this sound too stupid and naive on Trump's part? I don't think so. Josh Marshall of Talking Points Memo, who has played an important role in developing this story, has been very cautious here, as he should be, but what he hasn't done at all here is invoke what he calls "Trump's Razor." This is the proposition that, in trying to figure things out about Trump, one should always "ascertain the stupidest possible scenario that can be reconciled with the available facts."
Trump's Razor hasn't failed Marshall yet in this election cycle, and I don't think it would it fail him here if he thus invoked it.