I very much like designing games but I think I prefer to play them.
Metallum ... game I most proud of.
Does this only "count" as 1 extractor for the purpose of scoring? But not for the purpose of being destroyed by Action Modules or held in place by a Signal Jam?
Yes, you are right. That is why the mining station only counts as an extractor in the scoring phase.
Action Module Cards
When they state "Play this card immediately after purchase or at the beginning of the round", is this a choice you can make? Does it matter if you're going first or second? And do you have to play at the beginning of the next round or can you hold onto it until later rounds?
Yes, you get the choice. It usually does not matter if you are going first or second (for the rare occasions when it does matter please check the Metallum - FAQ 1.0 for exact ruling). You can hold the action module for latter rounds.
Does this include destroying your own extractors if your opponent no longer has any left or they are protected by Force Shield?
Yes! Military Satellite can be dangerous.
Again this card is worded such that you destroy your own extractors if you've got the majority. Is this correct?
Yes. You are correct, you need to be very careful with sabotage.
It states to choose any available Planetary Module and place it on a planet without any cost, however there are no available Planetary Module cards, only the Action Module cards that replaced them at the end of Sequence II. Is this a typo and is Acquire Module supposed to apply to the other Action Module cards on the market?
In case of Acquire Module you can look through all Planetary Modules (that are out of the game at this moment as you point out), choose one of them and place it on a planet of your choice.
Thanks for your questions and I hope my answers will clear things up. I am glad that you enjoy the game!