How can I use some Javascript to force a fieldset to stay open
You could definitely do that with some custom JS, but it might not be worth the trouble as there's a pre-built module which does what you need:
Fieldset collapser

Whenever a user clicks on a collapsible fieldset header to toggle the &quot;collapsed&quot; state, the new state is being saved - uniquely for every form fieldset possible. Depending on the user's being registered with your site or not, this happens either in his user account data or locally on his computer using cookies.
When the same form is called the next time, the fieldset states are restored to what they were last time. For registered users, this happens already on your server while preparing the form. Anonymous users will still see a little form animation by an invisible hand but then also be happy with their favorite fieldset states.