After reading all the threads you posted and researching about host mode you can tell most suggestions are for either the Arduino board or the mini breakout board. The suggestions are often applied to the specific baseboard and include most things you already mention in the description of the thread such as making sure you’re using an OTG cable, power the board correctly, make sure switches are in proper position, etc. For this specific Sparkfun board it’s hard to tell if host mode is possible. If more power can’t be applied through USB or externally then host mode won’t be likely to work.
Have you tried to contact Sparkfun support to see if they provide an alternative solution to use USB host mode?