There are species of seaweed that can grow up to 2m in a day. Pulling CO2 from the oceans might be a better long term strategy than pulling CO2 from the atmosphere (since oceanic CO2 comes from the atmosphere), since it's more heavily concentrated. Atmospheric CO2 capture sounds great until you realize that putting it on a smokestack is just inherently better, at which point it just becomes CCS.

Shipping is looking to methanol as the most likely future fuel for their vessels. Existing cars can be converted to run on methanol fairly cheaply and easily; the main issue tends to be cold weather starting. It's probably the most realistic way of reducing our carbon emissions, since both the hydrogen and carbon feedstock for methanol can be made carbon free (or even negative, depending on how you do it), the production process is already in place on an industrial scale, and it doesn't require everyone to fork out for a completely new vehicle.

Releasing carbon into the atmosphere is fine as long as in aggregate your fuel production doesn't use all of the carbon you pull from the atmosphere. When you do that, you're still carbon negative as long as what's left over doesn't oxidize. There are also a number of untapped sources for carbon. Sewerage, for example, contains carbon that was pulled from the atmosphere by a plant. Rather than letting it convert to methane, you can convert some of the carbon to methanol, and sequester the rest, effectively letting the carbon cycle work for you.