1. Not 100% sure, but I think you need 50% regardless. So no, 25% should not be enough.
2. Monopoly is based on total availability on the map. You can exceed 100%, for example. Anything that increase % or add raw number have no effect on needs for monopoly. Anything that create resources on the map does effect monopoly needs.
3. Same logic as 2, so no. The same logic apply to the Autocracy tenet which gives +100% strategic resources (no effect on monopoly need).
4. Same logic as 2 (no effect), and furthermore, they should only provide luxuries that are not present on the map. So they should never help with monopolies anyway (a monopoly only exists if there is at minima one sample on the map).
5. I don't know what version you use, but WLTKD should be based on random luxury, including ones you already possess. Anyway, I'm not aware of any more ways to reset WLTKD (4UC Spain can proc them with their zoo, and building Public works).
You can also choose a CS that is far away from Germany and spam emissary to ally it. Since Germany would be far away, it would be harder for them to increase influence. You can then try to press a sphere of influence on it. That way you have your very own WLTKD reset. Alternatively, if you can, you could try to get one example of every luxury via trade or conquest.
6. I didn't know they could reset. Maybe see if there is a timer displayed when you hover over it ? The VP devs tend to make information available whenever possible.