I answered my own question on the latter point. According to Grower Jim (link
here), Maranta Arrowroot can be eaten boiled, roasted, baked or fried. So essentially it's a fair root vegetable, and incidentally happens to excel at extractable starch production (to the point where it's better known for this than for its fresh-cooked uses).
Regarding Tacca Arrowroot, I scoured the net again searching high and low, and though I didn't find explicit confirmation that it
can't be used boiled, the information pretty much leads me to that conclusion. Plants for a Future (link
here) mentions raw and roasted consumption, but I think that thoroughly misrepresents its use (raw is said elsewhere to be medicinal and used sparingly) and its danger (with some sources claiming it to be lethal). Most sources stuck to the standard processing techniques as described
here.
Perhaps considering it a low-quality famine food is unfair, as it really does seem productive for starch, and has been well-used in its native range (and beyond it). Nevertheless, like bitter cassava, it definitely isn't a proper vegetable, but rather a starch source. I suppose I wouldn't mind having it around tucked in a corner some day, but it's not really a priority for me. With the very wide range of starchy crops available that
don't require processing (though they
can be ground into flour), I think it's best to stick with those for home consumption. The only processing starch crop that really calls my attention (for its sheer productive potential, and its alleged quality) is True Sago Palm (Metroxylon sagu), but that's a conversation for another time.