your briefcase would be classed as your personal bag [no different than how many airlines are operating now] and would not be subject to the 8kg limit.
8 kg/17.5 lbs kg is asinine as it is way too low. My usual carryon soft side briefcase often weighs more than that yet will fit under the seat of a CRJ-200. And nothing in it can be gate checked; it's full of electronics, confidential papers and personal items. I have no problem gate checking my rollaboard when the overheads fill up, but the other bag I will not check under any circumstances as there are too many valuables contained in it.