Look, this feels like the 5th time I've had and eventually won this argument on this forum, and there's a buncha threads I missed and more from before my time. I'm not particularly interested in doing it again, so this will be my last post here. There is no argument to be had that, by RAW, you can Overrun during a charge. You've laid out a perfectly reasonable house rule (one functionally identical to my own house rule, though I also grant players a +2 on their CMB check), but it is a house rule. Action economy precludes it and even if it didn't, per charge rules that Overrun makes no effort to be more specific than, you have moved TO the first square you may legally attack from. It's that damn comma. If the action economy works we could probably ignore the other reasons it doesn't work, but there's just no way around that comma.
The reason I bash this point so persistently is that I think it's vitally important to have a good grounding in what the rules actually say, and adjust them from there. Then you can tell your players exactly what you're changing (and why) before the game ever starts. If you make an incorrect assumption about the rules, it won't occur to you to bring it up. I'm reminded of the time a one-off GM told a Magus, in midcombat, that he couldn't use Arcane Mark with Spellcombat because, even though it was legal, the GM thought it was cheesy and stupid. Which caused the regular GM to remark "wait, that's not legal, is it?" and the whole table dissolved into arguing about Magi. The whole thing could have been avoided (or been less disruptive) if the temp GM had brought that particular ruling up at session 0. Just tell your players you let them overrun during a charge and they do or don't get +2. I don't really care if you believe its RAW or not, just communicate with your players/ask your GM on rules you know are controversial, before they come up at the table.