The reputable guy you talked to is incorrect, wider front width increases front grip because it accentuates the affect of the caster and will jack more weight to the rear, resulting in more lifting action on the inside rear wheel.
Narrowing the front will remove weight jacking and give slightly quicker steering response, but it does not increase front grip.
Other things to increase jacking would be stiffer front bar, more caster, narrower rear, or more seat struts.
As Ted said, it could also be driving. 6″ wider than standard is pretty far out of the box, you also maybe are so far off in the front that the kart isn’t going to ever cooperate. My first suggestion any time someone can’t figure out a handling problem is to go back to the baseline setup and start over. It’s very easy in karting to keep going down one chassis tuning path and ultimately never get what you want and become so far off the setup that the kart does wonky things and fixing it is impossible.