That's because the black in the texture underneath is the same color as the text. Try making the text a very dark grey instead of black.

Alternatively, make a copy of the text sheet, with all the same entities underneath the original, but no sheet effects. Then turn all the text on that sheet a different color, like a purple. You won't see it, as it's below the black text, but it will separate it from the black in the texture behind.

It was a corruption in the zip file. You wouldn't have been able to install the broken version in the first place. So if the setup worked, you've got the fixed version.

Yes, you are correct.

Yes, you could do that, but putting the split entity back together might be a bit complicated.

Instead I would copy the entity to a temporar layer, hide the original one, and then cut it down to the part you want to trace. Trace this pared-down copy, then delete the copy and show the original again. That way you don't have to bother about re-combining the original.

Is that the last section of the polygon you drew? It's possible that the section between the first and last node in the polygon is problematic for symbol control points to interact with.

To fix this, try adding an extra node on the polygon in that section, and place it on the same spot as the starting node. If that doesn't help, do send me the drawing and I'll take a look.