Hey Calderas! Sounds like an interesting scenario you had going there!
To your questions:
1. Does purposely targeting a shield create an Opportunity Attack option?
Personally, I would generally say no, it’s kind of like targeting the armor. I let people target shields at basically no risk – but there are different schools of thought on this in the HEMA world, and targeting weapons usually does draw an OA in Codex rules, so I’d leave this up to the GM. Basically for me in real life fencing matches against a sword and buckler fighter I’ll routinely beat on their buckler a few times just as a provocation without seemingly encountering much risk.
2. Is that opportunity attack against Passive Defense or can the sunderer try to block it?
So long as you have MP in your pool, you can always use them for Active Defense if you want to. One nuance – Active Defense can mean parrying or dodging / voiding.
3. If sunderer chooses to block opportunity attack does that affect his original sunder attempt? (it seems like it should “spoil” his sunder attempt.)
This is another GM judgement call but I’d say yes – IF the original attacker (‘sunderer’) uses Active Defense. If they ‘ignore’ the attack and relies on his passive defense, then I’d say his original attack can proceed as normal. This is actually the purpose of techniques like Mezzo Tempo and Contra Tempo, to interrupt the other guys attack and force them to spend time and attention defending themselves instead of focusing on their attack.
In a real fencing match, when you cut ‘long’ – targeting your opponents head or body, and they get inside your tempo and target your hand or something, you ‘shorten’ your cut, the Germans call this a sturzhau or kurz-hau. I teach my guys to do this to avoid getting their hands cut. It basically means your original attack is aborted though you’ll have a chance at a bind.
Overall I’d say you handled it right, usually there is some flexibility on interpretation of the rules and I like to keep things moving. In the case you described above, (if I parsed that right!) I’d probably give the original attacker a ‘Free Dice’ to attack again, since he generated the Critical Success. Another GM call on that one though.
As for 8th level guy vs. two newbies – yeah interesting scenario! Getting ganged up on with the Codex rules can be very risky. I remember one guy on our old forum described attacking his mid-level party with a large number of Kobolds, something they would normally almost laugh at, but with the Codex rules, as you get ganged up on you use up all your MP, so it gets dicey quick (pun intended) he said the party had a tough time with the Kobolds and had to beat a hasty retreat which they turned into a fighting withdrawal, making brief stands at choke points and then falling back in sections. They apparently had a great time with the encounter which otherwise would have been pretty routine.
Grappling, as you probably noticed, is also quite powerful. A strong person with some Grappling feats, especially if they are armored, can cause serious problems for less capable opponents by throwing them down, taking weapons etc. In one of our playtest campaigns we had a guy who was an alchemist and a good shot with a crossbow, but didn’t have a lot of other fighting skill and wasn’t very strong. As he leveled up, he kept getting more grappling feats. After about 4 or 5 sessions, he was quite a dangerous opponent, moving in close with a dagger, and a shirt of mail under his clothes that opponents usually didn’t notice until it was too late. He’d rush in and throw them down or toss them on their head. He ended up being one of the best fighters.