The rules for resolving the Arcane Bolt ARE the same as for Elemental Bolts (OB versus DB), the only difference is that this spell can only deliver a limited amount of damage. Changing how the damage is determined (and as noted, there are multiple ways of determining damage already in the game in other spells) is just adding variety, not "breaking from the set rules".
Another way of doing this would be to still use the crit table (whichever you choose) and just using the hits of damage. You still get a spell that doesn't do "crits" but is still handled 95% of the way the rest of the game is. The only thing different is to not use the crit description. With this method I would say it is a Small attack initially, unless you use the added hits of damage from the crit description, then stick with tiny and have the scaling expensive.
Look at the spells Drain Life and Harm -- Harm is almost exactly the same spell as Arcane Bolt, the only major difference being that Arcane Bolt is OB/DB based and Harm is RR based.
Exactly what I was talking about by saying:
(Qualify: I do not know every single little aspect of the game so far, but have read over a lot of it, and so far this is the only oddity I have found.)
