First, for SWAP_ON and SWAP_CTL, the third argument to swapctl(2)
   represents the priority of this swap device. Is a lower number a
   higher priority or vice versa?

hmm, this isn't noted in swapctl(2) but only in swapctl(8):

     priority=N      This option sets the priority of the specified swap de-
                     vice to N.  The highest priority is 0, second priority is
                     1, etc.

   
   Second, can this syscall be used as:
   
   	swapctl(SWAP_CTL, se, 0);
   
   ... in order to change anything BUT the priority of the swap device?
   (Like, say, passed a swapent device whose se_nblks value has
   changed?)


as you noted, "nope"