The President is essentially above the law

Sorry to be unclear. The main thing I mean is giving more law enforcement powers to congress and the marshalls which I believe act independently of the DOJ (but I could be wrong/have an incomplete understanding of that).

I think the state vs. federal thing is tough, because a single state arresting high-ranking federal officials would get hairy pretty quick.

I personally think the pluralistic executive branch, where different officials are elected separately could easily lead to more brinkmanship in US politics as you say. Additionally, under “normal circumstances” it would undermine some things I like about our system. For example I think the presidential veto makes a lot of sense because it avoids cases where a bill narrowly passes congress and the president doesn’t like it and will undermine it at every turn. Vetoes mitigate the problem of having laws that are backed up by so much uncertain enforcement that no one knows what the de facto law really is. I realize the irony of saying this when it happens so frequently in our system, but personally I think it would get even worse in the case where some officials had a ton of executive power without any say in legislation.