Personally, I prefer thinner inks, provided they are not prone to feathering and line bloat. Having recently refreshed my acquaintance with Higgins Eternal, I now remember why I stopped buying it. It won't produce decent hairlines, doing little better in that respect than the peat-based 'walnut' ink crystals. It's easily out performed by IG and properly diluted sumi-e inks. It does, however, feel nice going on the page, which I suspect is the inky equivalent of 'but she's got a great personality.'
It's possible. I do use gum arabic with my iron gall inks, and those are very waterproof.
That's interesting about the Higgins Eternal. I'm surprised it doesn't give good hairlines. It was one of the inks recommended in Michael Sull's Spencerian book. I know the John Neal Bookseller website advises adding in a little gum arabic to it. Have you tried that? I don't do Spencerian well enough to be the judge of Higgins for that, however. I'm still a beginner. Has Higgins Eternal changed at all since Sull wrote his book (my copy is copyrighted 1989)?
A bit of a late response, however, back in 1980 or so, we were using Pelikan's India ink at Graphics West because Higgins' ink clogged like mad and made really poor-quality lines. Comments at the time from other graphic artists ran to the order of "It's cr*p, schools make art students use it, otherwise it'd never sell." I can attest from personal experience that no Higgins ink will ever again touch one of my ruling pens, it made enough of a disaster that I wound up having to dis-assemble it and re-hone. If you've never done that, I assure you it's a major pain in the arse, and takes hours to do corrrectly.
FWIW, I'm in the process of rendering down a bunch of hickory* hulls into ink. So far, it's produce a pleasant brown which dries very slowly. I'm going to let it reduce more and then add some de-natured alcohol to bring the volume back up to where it is now.
*Hickory, pecan, black walnut, English walnut, and butternut are all members of the juglans group of trees.