If you have the pose player add-on you can use the tool "move" that pops up when you click on the sim and with that you can fine adjust the rider placement over the horse and you can also use this as "cheat" to pull out the leg from the horse belly ^^ this add-on is very useful in many ways since you can really do tiny adjustements, so small as 1px in time if needed to get the rider (or horse) in perfect position ^^
With wide horses you typically have to have one of the rider's legs partially clipping into the horse and take the picture from the angle with the other leg in the correct position, because most poses won't have the rider's legs posed wide enough to fit around them. The horse rig you use to create poses in Blender doesn't have any sliders affecting it, so most rider poses will have the legs in a fairly narrow position.
I can't say I recognize it, but there are a few mix and match tobiano sets out there now, so maybe the creator used a few markings to make that pattern? Is it possible they can send you their tobiano markings from their own game, then you can try and find it by testing them in small groups until you find the right marking(s)
Have you already downloaded Offaly's poses? I know she made a dressage and a jumping posepack but I remember there being a few others too; as I remember rightly some of them would work nicely for casual pictures and they weren't poselisted either. I also found these and these on the CC database, maybe those will work too