How to connect cross bracing on rotated posts?
Option 2 is the best of your suggestions. It's cheap and quick, but will be stable. Use any available tool (or trial and error) to measure the angle, and use lumber of adequate size to avoid crushing it behind the bolts. Also, use longer bolts as needed to get enough penetration into the post.
An option that you didn't mention is to simply extend the braces to lap past the posts a couple inches, then run your bolts in at the actual point of contact. For purposes of a playhouse, and assuming adequate bolt size, this will work just fine. As long as there's little to no gap between the brace and the post, strength won't be compromised. Just be sure to not get too close to the edge of the post where you'll bulge the wood and weaken the joint. If you pilot properly you can be within an inch or so.