]So, what do you think of Willoughby’s decision to visit Cleveland and his aggressive stance with Elinor?

That "rather of command than supplication" sums it up pretty nicely: he's used to having his own way and he means to get it this time too. He's never learned to command himself, of course. It's pretty scary, in a way. As you pointed out, we're not at all sure he'd let Elinor leave if she kept insisting on going, and there's not much help for her there. It gives us an ugly picture of what he might be capable of. We don't know that Willoughby has ever actually assaulted a woman---his good looks and charm find him plenty of willing victims, no doubt---but he certainly has little regard for the proprieties here, or for Elinor's wishes.

]I have to wonder if he would have made his dash to Cleveland had he known Col Brandon was also a guest. I doubt it.

So do I, in spades!! In fact, one of my strongest memories from my first read of S&S was knowing that Brandon could walk in at any moment and find Willoughby there. It was very suspenseful and I remember being both relieved and disappointed when the encounter was over. After the earlier mention of a duel I didn't know what to expect.