You're correct in that enchantment is not identical to ceremonial magic. In fact, the -1 per assistant rule is not only just for enchantment, it's specific to Q&D enchantment alone, not S&S. (It's also -1 for anyone within 10 yards, assisting or not).
Enchantment also differs from ceremonial magic in that you can enchant alone, whereas you _must_ have assistants for ceremonial magic.
Q&D enchantment allows mages to contribute as much energy as they're able -- there's not a cap per mage, or in total.
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Originally Posted by Magic 17
A lone caster is limited to the energy provided by his FP, HP, and one Powerstone. But assistants can contribute their own FP and HP as described for ceremonial magic. Assistants may also use one Powerstone each
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For that 100 energy limit, you're probably thinking of the rule for how long the enchantment takes -- the process takes 1 hour per 100 energy required to complete the enchantment. But that's not directly tied to the rate of energy input -- each mage only gets to contribute once, at the end of that 1+ hour period.
In the example with Tubbs and Hawthorne, they're repeating their Powerstone enchantment multiple times in one day, not supplying energy to the same enchantment process multiple times and resting during just one process. Powerstone enchantments stack to increase capacity, a rule detailed in that spell. The example cites 10 FP just as an assumption on the amount of energy they can contribute -- which isn't a bad rule of thumb assuming average stats.