11,000 of these rockets would reach Mars every minute, but they would change course the last day to avoid crashing into it
It sounds like there are some tight tolerances for a rain of rockets continually bombarding Mars. I don't think you could have more than a minute fraction do the wrong thing.
Once the ice is launched it make take months or years to reach Mars,
Related to this and preceding points — it seems like this plan commits not only the generation making the plans, but their children and grandchildren as well, to large capital costs and important coordinating actions, to prevent a fraction of the rockets from hitting the planet, and to make sure the ice cubes do the right thing. Is it ethical to commit one's grandchildren to a plan they weren't consulted on? There are a few circumstances in which it could be argued that such a forced commitment is reasonable; what is the circumstance underwriting the starting generation's committing its grandchildren to the continued terraforming of Mars?