Welcome to the forum, Andy.
Nice to have you with us.
Yes, I'm familiar with that problem. The first thing I did on mine was to change the serrated and plain edge blade positions so the serrated blade is on the outside and serves as a lock when the PE blade is deployed. I use the PE blade more so I prefer it that way. I hardly ever use the serrated one. As for the large flat head, regretfully no fix.
Not so many tool options in this position as it has to leave room for the pliers and any tools that you have in that place won't lock the handles once you open them as they themselves serve as the lock. I leave the large flathead where it is and just use the flathead on the bottle opener. Not such a big size difference and it isn't a heavy duty tool anyways.