Would a tray+rocker then work instead of a roller if we don't have one of those? It seems like it should give the same type of effect as a roller (solution continuously passing over). I had been using a horizontal shaker with a tray before. Maybe the horizontal shaker doesn't mix well enough with low volumes, but a rocker might.
This is an old post so I don't know if you're still watching it, but... I haven't had as much trouble keeping them at the right density. Could you expand a little on the details of your protocol? They don't seem to become senescent for me until a few passages in, even if plated at somewhat low densities (obviously not too low). There are so many different protocols though for isolating primary myoblasts so let me know the details first.