I check the maintenance on mines in a couple of my games and it looks roughly like the 2.5% I would expect.
The mines could be leftover ones in storage as well. Although I know that warhead/mine research after the first level lags a bit, depending on the overall situation.
The 100 mines limit is more of a game play thing, nothing would be worse than a player dropping thousands of mines at a location.
You'd have to edit Script_AI_Politics (line 2858) and re-compile the script files. The source files are available at my website.
Wiki reference for scripting:
http://wiki.spaceempires.net/index.p...ting_%28SEV%29