You should always hang drywall from the top down. Starting at the top makes it easier to create a straight, tight line between the ceiling and the wall. Any gaps that occur will be at the bottom, where they are easier to manage. You can either cut the bottom piece of drywall to fit the remaining space or cover the gap with baseboard trim. Additionally, starting at the top is physically easier, as you won't have to fit the top sheet into a tight space between the lower sheet and the ceiling, or try to balance the upper sheets on the lower ones while securing them.

Whether a drywall anchor can be reused depends on its material, type, and the method of removal. Plastic anchors, including expansion and self-drilling types, are generally designed for single use as they tend to break or deform upon removal. Metal anchors, such as toggle bolts, are more durable and may be reusable if they are not damaged during extraction. However, with toggle bolts, the toggle piece can fall into the wall cavity when the bolt is removed, making it difficult to retrieve. If an anchor is damaged during removal, for instance by having its head cut off, it cannot be reused regardless of the material.

There shouldn’t be any gaps between your walls and ceiling. These can be fixed with joint compound but that is only a fix, and it’s much better if your drywall fits correctly in the first place. Sometimes gaps can appear if your foundation shifts significantly but if this is the case, you have a much bigger problem than patching drywall.

You should do a little bit of sanding between adding coats of mud. Knock down any ridges or significantly imperfect areas through sanding, which should help you apply the next coat of mud smoothly. If your first coat of mud is pretty smooth after it dries, a quick sanding is all you’ll need. Don’t forget to wipe away any dust after each sanding process.