I'd say that Steve Avery doing it, is the simplest explanation. It's not nearly as complicated if you don't confine yourself to what the prosecution has said about the crime.
Most don't believe prosecutions narrative of how it went down.
As you said, it's not even a fact that she was killed that night. What real evidence do we have that even says that ?
Jodi's phone call doesn't mean anything imo. If Steve is in the garage and hears the phone ring, he could just go in and answer it, and come right back to the garage. So that aspect is not a big deal to me.
But I also don't think it's complicated to rape her in the house, move her to the garage and kill her via pillow on head (no splatter), move her to the rav4 to transport her to the fire (back it up to firepit), clean garage, move rav4 to location, gather tires or whatever for the fire, and go to bed.
Not saying I believe that's what happened, but that's relatively simple and doesn't take any great complication and doesn't require her to be taken offsite.
It's just a flow of events. Rape, Kill, move body, clean, burn, hide car.
I think Chuck/Earl/Scott/Tadych/ex boyfriend/roommate/stranger would all seem to add complication to the process.
What complicates this case is that they wanted Dassey to be the evidence and so they were using what they coerced out of his mouth to create a narrative that doesn't make alot of sense imo. Trying to frame someone could get very complicated. But it doesn't erase that the crime itself could have been very simple, right ?
I think the simplest explanation above makes sense. Add to my above theory that Dassey helps him clean the garage, not knowing what the blood was from, and then helping him with a bonfire and maybe seeing body parts in the fire.
Again, not saying that's what I believe, because there might be an even simpler explanation with a proper investigation of all these characters.