Here's a fixed version.

As Sue also noticed, the line for your polygon was double, so your actual polygon ended up being that o-width space between the "inner" and "outer" lines.

I split it open, then ran DELDUPS to get rid of duplicate nodes, then joined it again.

Yes please.

This is an effect of how the template is set up, the lower left corner of the map border is not on the snap grid.

I have no idea why the template was set up that way, but it looks a bit weird.

Anyhow, to work around this, don't place the grid during the new map process. Instead use Draw -> Hex or Square overlay after starting the map, and instead of clicking OK in the dialog, click select points instead. This lets you put the grid points instead of the grid using the map border as reference, and if you have snap on when doing this, it will align perfectly with the snap grid.

(You can also use the set snap grid option in the dialog which will then fix the snap grid, but the existing snap grid is lined up with the background, so it is better to use it.)

No, there isn't a dedicated mapping guide for SS2. SS2 primarily extends DD3 with more styles, and is used the exact same way as DD3 itself.

And as a dungeon style, yes, you would find it in the Dungeon category. Generally, the Floorplans one is for modern/sci-fi type stuff, to separate it from the more fantasy-style stuff in the Dungeons category, although both are floorplan-level maps.

The link on the main site seems to just link to the first video in the old series, letting YouTube guide you furter. I can't do anything about that, but I'll try to poke someone who can.

The link in the forum sidebar on the front page should take you to his channel instead.