Realistically 1-2 launches per day is the max. Starships are cheap to build but launch sites are not, and you can't launch every day because of the weather.
If you boil seawater slow enough, yeah hardly any salt would be carried off with the water vapor.
But undersea volcanic eruption? I'd imagine at least some salt would end up where the water vapor does. And at ~3.5% (weight % of ocean surface), that could put a fair amount of NaCl particles in the upper atmosphere.
In terms of global mean temperatures, which are a measure of how much climate change we are experiencing, the impact of Hunga Tonga is very small, only about 0.015 degrees Celsius. (This was independently confirmed by another study.) This means that the incredibly high temperatures we have measured for about a year now cannot be attributed to the Hunga Tonga eruption.
For those of you planning to read this to find some positive climate news. Don't bother.
The story above cites a confirmatory study for its ozone hole result and should also cite contradicting studies for its warming result.
There was a study out of NASA/JPL that made a correct and timely prediction of major warming a year in advance (i.e. arriving last fall)
It gave a forcing estimate of +0.15 Wm−2, *equivalent to half a decade of global human CO2 forcing* arriving mainly at 18-24 months after Jan 2022 (last fall) and lasting for 5+ years
But our planet (and with it, atmosphere) is hardly affected by humanity's activities adding heat directly. Instead it's GHGs that affect the inflow/outflow balance of heat from the sun.
The heat from gasoline you burn is quickly dissipated. But the CO2 produced has an effect for many decades to come.
