It's the install cache folder from the installers. I think Windows like to have those available for uninstall/repair purposes.

To start with the last question, no, you cannot get an endless grid. In CC3+, a grid is an entity, not just a display thing, and an entity needs proper coordinates to compute the lines.

I would probably just erase the old grid and draw a new one. As long as you create a new grid with the same grid size as the old one, it will fit perfectly. Use the select points option in the grid dialog to size it to your needs instead of filling the drawing area, and place the first point at 0,0

The problem is that everything is on the TEXT sheet now due to the grouping, so you'll need to put things back on their appropriate sheets.

See this FAQ entry:

My drawing tools are missing their textures (or show incorrect textures)

Not easily. While I guess technically CC3+ has all the tools required to accomplish this, this isn't a normal workflow the program is made to do, and hence, would have to be done manually, there are no quick shortcuts.

You can use modifiers like endpoint when moving the section to position it precisely, but once it is in place, you need to deal with all those contours. I assume you wish to have them as continuous contours, which means you need to reduce those polys to lines, chop out the pieces you don't need, and join them together again. Not even sure if Combine Paths will be happy with those contours with a huge number of nodes. As an alternative to chopping and cluing together, you could instead use the trace functionality of a drawing tool, and basically create a new contour by tracing the outlines of the pieces that forms the contour crossing the map pieces. This may be simpler. But no matter the method, each contour must be done individually.