Everyone has a different idea of openers as a lot can depend on what works for them, but try not to over think it.
I would perform them whilst practicing. Chances are they’ll be a couple of climbs on track, so go hard up those.
If you have the time and freedom then I’d be tempted to do them seperately. It means your on-course time can be dedicated to planning line choice, tyre pressures, areas to attack etc
Hm, well the more I think about it it I’d lean towards not doing a separate workout. Put in a couple hard efforts as you’re running the course and if you need more time spinning your legs afterward go for an easy lap or ride around the area. The hard efforts on the course will allow you to get a better race feel for tire setup, how hard you can push in corners, etc.
Unless you’re used to spending a ton of time on the bike each week, I don’t see much benefit to pre-riding and then doing a separate opener.
I do all my “openers” on the pre-ride course lap. Gently ramp up your effort to warm the body then throw in some harder efforts wherever the course allows. Structure is not that important but getting familiarity with the course, line choices, where to eat, drink and pass is. Know what the finish line looks like especially if you expect your position to be highly contested. Don’t be afraid to stop, hike back up and repeat sections that didn’t go smooth the first time or if you see a better line. Just ensure you don’t do too much in the pre-ride to leave fatigue for the next day’s race.