I'm sorry to hear about Maisie's possible diagnosis.
However - if it is kidney stones - they should be visible on the X ray. The "Wait and See" approach sounds to me like the vet is unsure and it may be that having her teeth done will sort the problem.
You may need to go back for another X ray in which case ask the vet to check there are no stones in either the utreter or the urethra
I have had a piggie with an advanced case of kidney stones and, even though as Wiebke says, the condition is inoperable - just as with heart pigs, the condition can be managed with an appropriate care plan and Fleure enjoyed many happy piggy years with her mate Grommie following her diagnosis
Unlike other recommendations to use fruseamide or other diuretics to flush out the kidneys and hopefully flush the odd small stone right out through the system - we decided against this because the stones were too large to enter the ureters from the kidneys and therefore in Fleure's case it might have accelerated kidney failure.
We tested a stone taken from the urethra, found it to be carbonate and tried destolit as per guinea lynx
http://www.guinealynx.info/forums/viewtopic.php?p=1235538
The care plan would include alleviating the symptoms (pain) and also looking into changing her diet to slow down the rate of formation (see other threads on Ca/Phosphorous ratio). It also might also be worth discussing destolit with your vet - print out the pages on guinea lynx and also this post if it helps in approaching your vet.
I'm very happy to talk further with either you or your vet if required although at the moment we are keeping all fingers and paws crossed here it is just Maisie's teeth.
x