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Can you only fast cast as a defensive menuver? For example, if magus A is about to be run down by knights on horseback in a trained group, can magus B fast cast to help him out?
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If you fast cast in a given combat round, can you still take an action and/or cast a formula spell on your turn?
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For a mastered spell, can you use multiple special abilities on the same spell at a time? For example, can you Fast Cast and Multiple Cast a spell at the same time. There is a magus in my campaign who likes to fast cast / multi-cast three copies of Pilum of Fire at once. He's a Flambeau... shocking, I know.
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Yes. Fast casting is simply reacting to some stimulus. Whether that stimulus is your own impending doom or someone else's doesn't matter.
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Check page 83 of the core book. A magus may keep casting fast cast spells until a speed check fails. I couldn't find a specific mention of being able to perform the originally intended action. I think that's probably best left to troupe decision. If the magus on casts one fast cast spell he can probably carry on. If he manages a couple he might not have time to complete his formulaic spell.
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I don't see why he couldn't. Spell mastery is all about becoming more and more proficient with a spell. It should be dangerous.
Flambeaus with Flawless Magic are Fearsome.
You must use a firm voice and bold gestures, and the mage may not exploit any other spellcasting options, as there is not enough time (arm5 corebook p.83) That's mean that he cannot fast-cast/multicast but nothing hinder him to multicast pilums on his normal round and throw another single pilum as a fast cast.
While you can certainly try for multiple fast cast spells in a round my recollection is that you cnat both fast cast and get a formulaic off in a round.
Of course my recollection is at odds with what others are saying. I'll hopefully get a chance to find the reference or find that my memory is faulty later today.
As I recall, fast cast spells just interupts everything to toss out a quick intervention to a situation... It doesn't affect whatever else you do in any other way than you having to concentrate on multiple spellcastings (15(?)+ on concentration check, I think)
On a related issue - how many (different) non-fastcast spells can one cast in a round? And before you say automaticly say 1 and leave it at that - what about the MuVi spells requiring that you cast them at the same time as the original spell?
I think those are special.
Pilum is an interesting "defensive" maneuver. I know I'd have to do some verbal dancing a bit with my SG on that, depending on the circumstances. 
normally a fast cast spell is a spontaneous version. If you have mastery that allows a formulaic spell to be fast cast, I'd allow the multicast to be added in. The mastery is already going aginst the normal rules for fast casting.
Read the section on fast casting (pg 83), you can keep casting as long as you pass the difficulty check. If I recall correctly the difficulty starts at the iniative you want to beat for the 1st spell and goes up by 6 for each additional spell. You also get a -10 to your casting total.
I beleive you need to cast them simultaeously. which means you need to make a concentration check.
there is the normal fast cast defense where all you try to do is alter the spell in some way that it doesn't work correctly so all you need is ~1/2 the level of the spell being cast at you.
The other defense is to hit the attacker 1st and hope you mess up their concentration enough that they cant get the spell off.
I do not know whether fast-casting has to be "defensive"; it is simply a reaction to something.
So why can't you react to "Hey, look! A target!"
Fast-multicast "Twist of the Tongue" upon coming into earshot of complaining peasants? 
p174
So if you fast cast, you can not also cast a normal spell, only other attempts at fast casting.
Yes, but can you Fast Cast a Multi-Cast spell?
I Read it as.
Round 1
1 Fast cast spell maybe more depends on your quickness.
And
Only 1 Normal Spell in you time in the initiative.
Round 2
....
the sentance in question:
"Thus a magus can not cast more than one normal spell or a fast cast and a normal spell in the same round...."
"Thus a magus can not cast {thing A} or {thing B }...." looks to me the way it was intended.
Am I correct in understanding your interpretation as: "Thus a magus can not cast more than {thing A}or {thing B}...."
I see that we've come across a genuine case of ambiguity unlike what normally pops out of internet discussions where a clear statement is missread by someone who can't bring thimselves to believe that their position could be in error.
(At least) One of us should check the book to see if context makes it clear and if it doesn't, we should email David to have him make an update to the errata.
A semi colon in one place or the other would clear this up nicely.
I think an example would be better.
A Semi Colon could still be debated, and always will be.
Yes 
That would be nice. 
"Thus a magus can not cast; more than one normal spell or a fast cast and a normal spell in the same round...."
"Thus a magus can not cast more than; one normal spell or a fast cast and a normal spell in the same round...."
You're right an example would be better
So much words for a semi-colon 
Maybe some commas and some coordinating conjunctions?
"Thus[color=green], a magus ca[color=green]nnot cast [color=green]either more than one normal spell[color=green], [color=green]nor a fast cast and a normal spell in the same round...."
"Thus[color=green], a magus ca[color=green]nnot cast more than [color=green]either one normal spell[color=green], or a fast cast and a normal spell in the same round...."
I've read and reread the passage so much that right now I don't know if my edit helps or not. 
The issue is all about what's grouped with what--a common issue in creating clarity. I'm an English teacher, and I actually like these kind of issues. They tickle me. However, unlike David and a number of the other authors, I am an American with an American usage and style, so I intrude with commas. I don't think a semi-colon would pull it off in any case... maybe a colon in the place of "either," but it's a stretch for them two dots.
These are better but I believe you want or rather than nor in the first one.
"Thus, a magus cannot cast either more than one normal spell, or a fast cast and a normal spell in the same round...."
or I suppose you could say
"Thus, a magus can cast neither more than one normal spell, nor a fast cast and a normal spell in the same round...."
I started with those wordings--the more traditional style choice in your wordings lends a tone of ease. However, although we often pair neither and nor, as well as either and or, we don't have to, especially because of the not. Also, I felt "can cast neither" wasn't as lucid as the other wording: it distances the negative from the verb just a touch more and strengthens alliteration in a place that it can be distracting. Further, I retained nor to stress the negative, as that seems to be key to the meaning of the sentence. All in all, any of these wordings are acceptable in terms of (American) grammar and usage--at this point, it's a stylistic choice of what is emphasized. If it works, it's good.