I will assume you talk about CO2 adsorption at 273 K (or 300 K) and N2 adsorption at 77 K.

What you say not correct because N2 can also access micropores. The advantage of CO2 is that it can access pores even smaller than N2, or where there exists some diffusion limitations (CO2 adsorber at higher temperature, so it can enter the narrow micropores more easily).

You have to consider that the surface area measurement in porous systems is NEVER absolute, but it depends on how yo define it and how you measure (the molecular probe).

The total pore volume can be read from P/P0 near 1, but without reaching saturation conditions (say, P/P0 ~ 0.97).

Why don't you have a look at the classical paper of Paco? (see below)

Use of nitrogen vs. carbon dioxide in the characterization of activated carbons

J. Garrido, A. Linares-Solano, J. M. Martin-Martinez, M. Molina-Sabio, Francisco. Rodriguez-Reinoso, R. Torregrosa

Langmuir, 1987, 3 (1), pp 76–81

DOI: 10.1021/la00073a013

Greetings, Fernando. Indeed you assume correctly, I talk about that.

I have read the paper you provided and also the book by Marsh and Rodriguez-Reynoso.

Indeed my asumption that CO2 sorption is usually larger was incorrect and is highly dependent on the nature of the material; however, for biochar this does seem to hold.

My understanding so far is that Interpretation #2 is the correct one, with the modification that "total" SSA is whichever comes out larger, as long as no hysteresis appears during desorption, but the narrower < 2 nm are still being probed only by CO2.

No: N2 can still access most micropores.