Mostly, yes... You don't necessarilly need to turn CO2 back to hydrocarbon, there may be less expensive ways to capture and store that carbon. Still, you're correct that the costs will be gigantic.
The energy costs are largely in the sequestration of carbon from the atmosphere, or other biospheric sink. (Seawater is actually a highly viable reservoir and sequestration source.)
Once you've got the carbon, what you turn it into is relatively academic. Though if you do turn it into liquid hydrocarbons, it's 1) very long-term stable and 2) can be used, after the atmosphere returns to normal, which could be some centuries or longer, as fuel in dedicated applications so long as the net carbon cycle is appropriately managed.
It's got to do with the volumes of carbon we're currently emitting, and which we have emitted.
Keep in mind that the carbon cycle was already in balance before humans started digging out gigatons of coal and pumping gigatons of petroleum. We've skewed that balance -- not by a terrible lot (or we'd have been in trouble a long time ago), but by enough.
The problem now is unwinding ~100 years of industrial fossil fuel use, quickly.
Remember that while plants fix carbon, it doesn't necessarily stay fixed, and you cannot release what you're fixing from the atmosphere if you want this to work.
Trees and plants don't fix CO2 "for free", they have a considerable energy budget they expend for this, and have other things to tend to as well (making leaves, drawing up water, fighting off disease and insects, feeding animals).
So you're talking about increasing plant growth beyond present levels, and preserving the carbon without allowing it to be re-released to the atmosphere for at least a few centuries. That's a tall order.
Humans fixing CO2 requires energy to do so, energy which will have to come from some non-emitting source (solar, wind, geothermal, and nuclear are the only really viable options, hydro, tide and wave are too small to matter), and will compete against other human energy uses. Renewables have their own considerable energy input requirements as well for construction and maintenance, which multiple credible sources find close to the limits which allow technological society to happen (see Charles A.S. Hall's work on EROEI, he's got a book in progress due out next year I believe on the topic).
It depends on the timescale you want to operate with.
Also, you'd need to reverse the topsoil destruction and the spread of agricultural exploitation, which is not going to be easy when the population keeps on growing.
