Yeah, you don't need a transition matrix for a simple case like 100% DA and max 2 hits per round. Letting D be the event that a double attack occurs in a single round, it follows that the probability of overflow is
http://latex.codecogs.com/gif.latex?...+ P(D|DD)P(DD)
and the joint probabilities are straightforward to compute from independence of events.
How would you account for situations with non-100% hit rate and like 5 hits possible per round, though? It's easier to set up the appropriate transition matrix, accounting for how much overflow might occur (1, 2, 3, or 4 hits in excess) along with the probabilities of 0, 1, 2, 3, 4, and 5 hits. Then obtain the appropriate n-step transition matrix to get the probability of overflow for any x-hit setup (limited only by the matrix dimensions). You can make the transition matrix as large as necessary to encompass dual-wield scenarios.
Example: 95% hit rate, dual-wielding with 15% double attack rate and 15 hits required to attain 100 TP. Obtain the overflow probabilities (probability of ending with 16, 17, and 18 ) from a 14-step 19x19 transition matrix (you don't really need to go to 14 because probabilities quickly "converge"). They are .4246476, .11025272, and .008391217. The average number of hits is 15.67033, which agrees with an alternative method of obtaining it (brute-force conditional expectation).
You say you have a formula for Love Halberd, but do you know how it really works? If it's like Fortitude Axe, it won't quadruple-attack. If you know that already, are you sure you know how DA interacts with it (where applicable)? If you're wrong, then you would have to go back to the drawing board. Alternatively, you could just crank out the appropriate transition matrix without deriving hard formulas.
NIN DPS: DPS figure follows from the concept of a rate.
OAT and DA: OAT great axe is junk if OAT and DA work like Joyeuse. We'll see.