So they are used to in at night/out all day and you want them out 24/7?
As long as they are happy when out then no reason not to do that. As far as a routine just be consistent. I don't like my horses on too much of a schedule (they get psychotic about it I find and I like not having to stress about it) but in this situation I would definitely pick some times and stick to them. Bring them in at 7 am, feed, leave them in for x amount, brush and turn out. Bring in at 5 same as am, Check them at 9 and bring carrots. Something like that, just be consistent and be around, I doubt she is annoyed, maybe just a little stressed at being "forgotten". If they are happy outside that's what you need to worry about and it sounds like you have good horses for it, some are spoiled little twerps that can't adjust to being horses yours will be just fine ;P.
I always keep mine in at night out during the day but when I moved out my mother didn't want to deal with the stalls either so now they come into the paddock near the barn at night then pasture during the day then feeding am and pm they get locked in their stalls for a few hours. Stalls are always open for shelter. They are all just fine and the two that didn't like to be shut in are much happier, the one that has ALWAYS been in a stall at night his entire life is also just fine. I do like them in at night but they are fine, and sometimes better for it out.