Yes and no. Meaning: depositing fertilizer close to the shore in shallow waters causes toxic algal blooms and suffocates everything. One needs deep, cold waters so the sinking organic matter is not decomposing rapidly.
And yes, if the winds blow the Saharan dust over large oceanic surfaces algae do grow in otherwise "desert" waters and some of the generated biomass does sink deep.
One aspect these iron fertilization experiments have ignored is albedo. Leaving any sequestration on the table, and just focussing on keeping the albedo of equatorial oceans low for as long as possible has the real potential to avoid absorbing massive amounts of solar heat into the system. That doesn't solve the co2 problem, but because you can negate the feedback loop aspect of the problem, and perhaps even make natural sequestration occur faster if you lower temperature enough. One thing I haven't been able to figure out from the data from the experiments, is how long these blooms last. The tonnage of fertilizer and the size of the bloom, yes. Also, the experiments were done in eddies mostly to isolate the effects. So an open equatorial ocean experiment would be needed. Along with better local temperature monitoring. Some satellite imagery to verify albedo would be a plus. As far as ballpark numbers, if the bloom stays equatorial, you would need about 1.5e9 USD per however long a bloom lasts in order to maintain an albedo that would initiate snowball. That is scaling linearly up from the area affected by previous experiments vs their costs.
From German Wikipedia the albedo of water changes a lot with the angle. Oceans already are "dark" when hit by sunlight at +45degs angles. So it may not be that important.
And yes, if the winds blow the Saharan dust over large oceanic surfaces algae do grow in otherwise "desert" waters and some of the generated biomass does sink deep.