I have yet to see any evidence that CHR significantly impacts critical damage. Then again, you have no reason to believe that BLM will not get more crit/less crit than SMN. Furthermore, pretty much all stat weights are like, <1% difference, so it is not significant. A SMN may have 1% more crit than a BLM.
Secondly, yes you could tri-d at the end.
You could also deathflare at the end. Similarly, a BLM can fire 4 or whatever at the end. Both classes have burst windows, etc. I am not saying that you are incorrect, merely that DoTs are impacted just as much as anything else, no more, from buffs.
And yes, Tri-D if inopportune will cut into its effectiveness.
Saving contagion if it did not line up with your other CDs (like dreadwyrm) also means a possible dps loss if you end up using contagion fewer times during the fight. Also using garuda in general has not been shown to be an actual dps gain. I would posit that using garuda on any fight except one where ifrit will simply die (pretty much none, if you micro him) is a loss.
Contagion, again, can be thought of as simply a nuke, if you use it while dots are buffed, its like you cast the nuke during the buff.
Again I should also state that 15% crit is a really, really minor buff on top of everything else. It isn't worth saving everything for, though it does nicely sync up with raging.