Given that under the rules for Additional Attacks (pg. 84), the number of attacks one gains during a combat round is based on the Final Attack result (d100 roll + modifiers) of the character. Under that rule, how then, in the Counterattack example, was Celia able to declare to the GM that she would be making 2 attacks in the combat round?
It's not Final Attack result. It's misleading. It means Final Attack base score, before rolling the dice.
If you have less than 100 Attack (including Characteristic modifiers, Martial Arts bonuses, etc), then you get one attack per round.
More than 100 but less than 200 is up to two attacks per round (but you pay a -25 penalty for both attacks that round if you attack twice). The penalty can be reduced by certain skills (like the Martial Art Kempo).
More than 200 but less than 300 is up to three attacks per round (-25 to both if you attack twice, -50 to all three if you attack three times. The penalty is -25 for each attack additional to the first one, and they just keep stacking).
And so on. Celia, in the example, has Attack 120, so she can choose to attack twice, but both attacks are based on Attack 95 (120-25).
When Counterattacking, it is assumed you make a standard attack roll, and add the Counterattack bonus given on the Combat Table. Is it possible to use the rules for Additional Attacks on Counterattacks? For instance, is it plausible for a character to build up his Defense score to increase the chances of Counterattacking at a greater bonus, and then use that bonus to potentially increase the number of strikes he can make with that Counterattack?
No. Counters work like any other attack in that sense.
Firstly, the number of attacks, including counters, is defined by your Attack score (Celia's 120, above).
Secondly, you can't get more than one strike off a single counter. The counter attack is only for the attack immediately after a successful defence. Any other attacks are just…. attacks, and don't get counter attack bonuses.
Thirdly, you can't build up your Defence too high at the expense of your Attack. There is a rule that your Attack and Defence scores cannot be more than 50 points apart before you roll. So if you push your defence up, your attack will eventually need to follow it.
Fourthly, there's not a lot of point, except in very specific circumstances (one of which I'll get to in a moment) in doing what you're suggesting, other than simply "staying alive", which I'll grant you is important. Because the counter-attack bonuses: a) only add to your base attack score, so all they'll be doing is bringing your Attack back up to nearer your Defence; b) don't kick in immediately (you need to beat the opponents attack result by at least 11 points to get any sort of bonus to counter), so you're better off having a higher attack to get a better overall score on your counterattack, c) it means you're dependant on not only beating their Defence with your Attack roll, but also significantly beating their Attack with your Defence roll to generate a counterattack bonus (and with two rolls, there's an increased chance that either one of their rolls will be good, or one of your rolls will be bad).
But all is not lost….
The character is more than likely going to be starting off as a Tao to get a good blend of Martial Arts using the rules present in Dominus Exxet (most notably, Xing Quan, which will be used to obtain the Advanced Martial Art Mushin, and Kempo, to reduce the penalties for making multiple attacks in one turn). I'm also trying to find a way to swing the Capricorn (Needles) Impossible Weapon in there, and possibly switch classes to Technician after a few levels for some basic Ki Techs.
Mushin can take a while to get to, not least because you've got to get a pretty good Medicine skill to achieve it. But by all means, go for it. It's a cool MA.
But if you're set on the counterattack plan, try and get Selene (or if your character is a man, create a male version of it…. you're allowed to just rename it as something else and say it's like Selene, but isn't exactly Selene. It's all about where that specific MA comes from.). Selene at Base Level gives you double the counter-attack bonus. So suddenly, your strategy starts to become more effective. Selene at Arcane Level gives you infinite penalty-free counter-attacks in a round as long as you keep making successful defences. And you can still do your normal attacks on top, because the Arcane Selene counterattacks don't count towards your maximum number of attacks/active actions in a round.
You'll need to get Aikido to get to Selene, but from your strategy, it would seem the ideal Martial Art. Plus you can always combine the benefits from it with the effects of Mushin (and any other MA you have).
Other Advanced Martial Arts to consider for you are Dumah (because if your attack isn't that good, he can block you, but you might break his weapon when he does so, so his next attack will probably be a lot worse), and if you can get to Arcane Emp (you need to get over 300 Attack, so this is a looooong way off), that could also really help, because every time you successfully defend against a weapon you have a free chance to disarm him. Also, just in case he DOES get through your defence, Rex Frame. It's totally penalty-free armour.