*Hug* You are as much of an overthinker as I am, you must be exhausted from all that wondering!
I'm guessing other people have much better ideas about how to talk to Sven and your husband. Ordinarily I wouldn't recommend not bringing up "poly" to a SO, but as Thaddeus already knows you have feelings for Sven, and you're not in a rush to be "IN" a formal poly relationship, and you know your husband best, that sounds like a reasonable way to go, as long as you don't think he'll feel misled.
Sept. 8 seems a long way away. Phew, it is heavy work when you are the partner that initiates all the difficult conversations. In your shoes what I might do is write down what I want to know/clarify/talk about to Sven. Pick the top one or two in importance to me, and focus on only worrying about how I feel about those things too until I can talk to him in person. Especially if you're finding your mind drift to all these questions whenever you have a free moment for idle thought.
If all those things you just wrote about possible scenarios are circling in your head day after day though, I would probably write to Sven in a few days and ask for clarification on whatever is keeping you distracted.
I try to operate by being myself as much as possible. It is not easy, but if you are being tortured by what's happening and you say "Hey Sven, am I cut off?" well either you get to find out and can relax, or he'll hedge or be dishonest but you'll figure out that sooner or later. It doesn't sound from the tone of your posts that you are unhappy with these thoughts, just curious about all the damn possibilities.
Me personally I don't think I'd want that gift horse, if you think he's out of there as soon as he finds a full time relationship of his own, that's going to be painful, but at least it seems to be a risk you're willing to take. I'd probably say "Sven, I'd like to have you in my life long term, and think a relationship like this would look like _______ " What do you think? But that's true that it's scary to say, maybe scary to hear, and I suppose if you would like it to last as long as possible and you think it's going to have to end sooner or later, maybe it's safer not to say it. Then again, if you say it and he's open to it, that can open up the chance for greater rewards, if he wants to be with you long term too maybe, and doesn't know it's possible, it could change his view on what else he is looking for in life, and give you the chance to build something really nice.
p.s. my example of bravery I'd exhibit in telling Sven that I wanted him in my life long term are over-exaggerated
Also thanks for making up fun full names instead of using initials!