1. As far as I can tell, yes.
Quote 1, From the CRB: "To cast a spell with a material (M), focus (F), or divine focus (DF) component, you have to have the proper materials, as described by the spell. Unless these components are elaborate, preparing them is a free action. For material components and focuses whose costs are not listed in the spell description, you can assume that you have them if you have your spell component pouch."
Consider the acid splash spell . It doesn't require a focus or material component, but you can use an acid flask as a focus component to get +1 damage. This item implies pretty explicitly in my reading in addition to the above text that you can plop a focus component on a spell that doesn't have or require one regularly.
2. Given it's a free action to prepare them for usage as you cast the spell, and this quote: "You can perform one or more free actions while taking another action normally. However, there are reasonable limits on what you can really do for free, as decided by the GM.", I'm inclined to say yes (within reason of course)
3. Yes, using the quote as a reference point. For example, dweomer's essence. You have the option of using it as a M component to boost your CL checks for spell resistance.
4. Yes I believe with the same rationale as explained on #2.
1.) You can use a focus component on a spell as long as an item says you can use it as a focus component. George gave a fine example of a time when you can add a component.
2.) You can use a focus as a free action, so yes you can use as many as you want. Keep in mind you still have to manipulate the focus though.
To cast a spell, you must be able to speak (if the spell has a verbal component), gesture (if it has a somatic component), and manipulate the material components or focus (if any). Additionally, you must concentrate to cast a spell.
3.) Yes you can as long as it says it can be used as a component. For example check out the varisian idols and how they can be used.
4.) Yes based on answer 2.
Nice, can I use multiple of that same type of focus then?
While you "could", I don't believe it'd have any practical effect, as benefits from the source don't stack unless it's special and says it does.
Tl;Dr no -200 penalty with void shards for you sir!