About the second question.

2 is the limit for drawing tools. Other than that you will have to manually copy them to other sheets.

I've used a similar method in my DD4 example map (sorry - can't show you that yet) - not Color Key, but a sill sheet. I have sills that match the walls. The windows and doors only cut the walls. When the sill appears in a doorway it looks like a threshold. The trick is to not put anything more than a slight glow on the sills so the eye interprets it either as a windowsill, or as a threshold, depending on context.

A note about the scale of a Texture Overblend.

The scale of this effect is tricky to get right. In the map the red tundra fill has a scale of 125 miles. I had to find out by trial and error that the correct Texture Overblend scale to set was 0.065. It's usually way smaller than the default 1, which will give you just a pixelated mess.

The texture overblend is shown in the rectangle to the left of the dialog.

I'm not sure. In the case I used it the fact that Blur, Alpha will give your poly a fuzzy edge without the EFI didn't matter, because it was immediately followed by an EFI.

The only way to find out is to try it.

Another way to use the same fill over the top of itself is by not using the fill at all, but adding it to a plain coloured poly using a Texture Overblend. I think that works. I can't remember the results off the top of my head. I think I noticed that as a side effect of something else I did.

Remember, though, that every fancy sheet effect you use requires processing time, so there are circumstances where a backing sheet might be better than adding Blur, Alpha, or Texture Overblend.

Add a new sheet above the roads (below them in the list order on the Sheets and Effects dialog), add a Blur, Alpha, and then an Edge Fade Inner sheet effect to it, and then draw a patch of the background texture on it over the end of the road. You will have to adjust the EFI until you get the desired fade length.

I use the Blur, Alpha to prevent Transparency Acne, which is an undesirable side effect when using the same fill on top of itself with an EFI.

Here I drew a 'road' of lava texture on my BACKGROUND land in an overland style, and then added BACKGROUND 2 with the necessary sheet effects to add the patch of more land texture over the end of the road.

I think you should be able to use MIRCPY. It requires that you click twice to define a mirror line on the map. You will have to delete the original and move the copy into position.