I’m guessing there’s a butterfly/ wingnut on the lower section of the bolt, so your molgrips should be ok.
as for the top nut, I’d invest in a set of box spanners. You’ll find other uses for them.
assuming that you can find the correct size, they should give good purchase on the top bolt.
its easier/ less swearing to clamp the bottom nut while you loosen the top nut.
there must be a limerick in there…
after many sittings, you may notice corroded fittings
these may be in an awkward site, near the place where you …
An investment in some tools to maintain the vortex of your stools
will ensure much less cursing and less knuckles to be nursing:)