Yes, you're right. The Queen has to give May the chance of getting a Speech through as leader of the largest party; if she can't, then she has to give Corbyn a go. If he can't either, then it's another election.
I can see that May would want to avoid this at all costs, though. Which means that irrespective of what happens with the DUP, the Queen's Speech is likely to be pretty bland and contain only measures with which the other parties are unlikely to disagree (some sort of regulation of energy bills for example).