ok.

I had the impression that a sustaining focus was sustaining the spell for you, and that it didnt require any attention at all : allowing you to cast exclusive spell without dropping the sustained spell

If you have spells being sustained by sustaining foci, you do not have +2 TN per spell (which you normally would for sustaining spells) because the foci are doing the sustaining for you. You can also perform exclusive activities such as summoning and projecting; if you didn't have sustaining foci you would have to drop sustaining spells to be able to perform an exclusive action.

Activating a focus requires an action so you can not use a sustaining focus to sustain an exclusive-limited spell because you are concentrating exclusively on casting the spell and so cannot simultaneously activate the sustaining focus. This is explicitly stated in the book. (This goes for a spell learnt as exclusive for either reducing the karma cost of learning the spell or the TN of resisting drain.)

You can however cast an exclusive spell while a sustaining focus sustains another spell.

Generally when considering limiting spells, it is good to use 'exclusive' for instant spells because you're (virtually) never going to be casting more than one at once. And they are often combat spells with variable damage levels so if you can reduce your drain TN by 1 that's a good thing.

Fetish limitation is good for spells you're going to sustain especially at force 6 (or any even numbered force). Since fetishes and foci are made using the same process (and fetishes are far simpler and cheaper than foci) a sustaining focus can itself qualify as being the fetish for your fetish limited spell. If your spell is cast at force 6 (17% chance per die of being successfully resisted) you resist drain as if it was force 5 (ie your resist TN after division by 2 and adding modifiers is one lower). Don't cast a fetish limited spell at an odd force level; eg casting at force 5 (33% chance per die of being successfully resisted) you resist drain as if it was force 4... and 4/2 == 5/2 so you get no advantage on the drain test but your victims get a significant increase in their chance to resist.

You sure can, but as you later discovered, the exclusive limitation is beneficial for odd or even force spells whereas the fetish limitation is ineffective for spells with odd force. And increased reflexes is a spell you typically want to sustain. If you want to make its drain easier, and you are quite happy to keep your power to yourself, you can learn a modified version of increased reflexes +3 "caster only". This means you can't boost your chummer's reaction (but then, he's probably got cyberware reaction boosting anyway, or an adept power, edge, etc etc) you can only boost your own reaction but the drain is much easier.

There is a minimum TN for all tests, including resisting drain. Limitations and spell design effects can reduce drain a great deal but in the end you roll against a minimum of 2(Light). Even if you were to cast some hypothetical spell at force 1 with the exclusive limitation and and it was a -3(-3) spell you would still resist 2(Light) drain (so if you cast this particular spell at or below 10(Deadly) you'd still only have to resist 2(Light)).

So for your stunbolt example, ((4-1)/2)-1=0 as you say. So if you cast 4(Moderate) that would work out to be raw drain 0(Moderate) which you would have to resist at 2(Moderate) since the minimum TN is 2. In which case, there is no point casting the spell at force 4 or lower (since you're going to be resisting drain at TN 2 whether or not you make it easy for your victim to resist the spell). If you cast it at force 6, ((6-1)/2)-1=1 or at force 8, ((8-1)/2)-1=2. So if you can cast it at force 8, do so - it will be much harder for your target to resist but just as easy for you to resist as if you'd cast it at force 1.

Yes there is a minimum. In any given test the minimum Target Number is 2, as a 1 is always considered a failure. (Correct me if I'm wrong anyone)

IIRC the -1 for the fetish (or -2 if it's an exclusive spell) is subtracted from the spell force before you divide it by 2, then you round down if needed and then the +/- from the spell drain code is calculated.