Or just type the coordinates on the command line. Place the first corner normally with the mouse, then enter @70,15 on the command line for the second corner.

You should be able to get them through the archived version of the site on archive.org.

I think their archives for update 2 are corrupted, but you only need update 3, it is a cumulative update.

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.