Assuming you use the rules in "Hucksters & Hexes", is Hexslingin' no longer an aptitude? "Hucksters & Hexes" makes it sound like each hex is an aptitude. Is this interpretation accurate?
No. In fact, it's the opposite.
In the original Deadlands rulebook, each spell was its own Aptitude, to be bought up individually. That rule was changed to the Hexslingin' attribute for Hexarcana and the Deadlands Player's Guide. So you're right to keep using Hexslingin', as that's the newer rule.