When you need to to a precise export, make sure that you only export the map itself, and not any border/screen, as this will mess up your calculation. This is typically what causes the drifting you describe since then borders steal some of the pixels you calculated for the grid cells, they suddenly aren't 50 pixels anymore. Use the save as rectangular section to accomplish saving just the map. Make sure that the export size is calculated correctly, for example, a 11 by 12 squares map sghould be exported as 550 by 600 pixels to fit with 50px/square

Also note that 50 px/square does not give you a very high quality result if you like to zoom in as in your image above. I recommend at least 100px/square, MapTools handles that fine (Of course, that also means that the players get sent a bigger file, so you need to weight quality against bandwidth/transfer times here)

Generally, I also prefer to leave the grid off from my CC3+ export, as it generally isn't needed. That also minimizes the effect of drift a bit, but again, drift won't be an issue if the export size is correct.

Maybe my blog article here can also help you a bit:

Also note that the change properties dialog defaults to the currently active settings as shown on the status bar, they are NOT showing the properties of the currently selected entity, so nothing is "reverting back" here.

For easy ways to place those symbols, either set up a snap grid matching the size of the symbols, or you could probalby get them placed on a line with Symbols Along using the appropriate parameters.

The wave fill is defined in each template, so you'll need to edit it for every map you want it changed in, but what you can do is to go to Symbols menu -> Symbol Manager, then check the box that says display fill style symbols. You should now find the Wave fill symbol in the list. Select it, and hit edit. Define the two corners of the editing window as CC3+ asks you to do, and then simply use :CC2MCHANGE: on the waves and make them black. Close the editing Window, and say yes to keep the changes.

Well, if you just want to use that zoomed-in portion as it is, then all you have to do is to use pick Rectangular section JPG when you save the map, this causes CC3+ to prompt you for two opposite corners that define your export area instead of exporting the entire map, this allows you to export arbitrary (rectangular) portions of it.