Any suggestions what would be the best way to add road closures/detours in Garmin Mobile PC?

There are at least a three or four ways, depending on what you really want to do.

If you are following a route and encounter a detour or road closure, just follow the signs and Mobile PC will recalculate a new route on the fly. If the signs push you away from the recalculated route, that's okay. Mobile PC will recalculate as often as it needs to to get you where you were originally headed.

If you force it to recalculate frequently, any slight annoyance you start to hear in the "Off route, recalculationg" prompt is entirely in your head.

When you are creating a route and you know there is a place, or places, you want to be certain to go to/through or you want to avoid, be sure to include those places when you create the route. That is, if you want to go to/through them, include them. If you want to be sure to avoid them, pick some place(s) along the way that will be guaranteed to take you around. Once a route is saved you can edit it to insert additional points to take you through or around, or you can "shape" your route.

If you want to establish permanent avoidance areas, use the SETTINGS > ROUTING feature. You can add a road or you can add a rectangular area.

I hope that covers what you needed.

...ken...