Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
not an expert here, but I would say not under those circumstances. How long are they at those temperatures? How thick is the ink deposit?It would be nice to hear something from the manufacturers though!
They appear to not be cured sufficiently.Especially with small dryers you may find that a few inches on the edges of the panels are not really useful, ie, don't plan on curing an 18" wide print on an 18" wide panel.It is not uncommon for shops with big prints and small panels to run shirts through multiple times, each either rotated or shifted to one side or the other. When you find yourself at that point more too often, it's time for a bigger dryer.Drafts can add to this problem.In answer to your first question, yes, you can over cure plastisol, but you aren't doing that here.
Also, another thing, it seams that the other design on a black shirt printed with 156 mesh feels a bit better, I still need to do wash test.