As someone who gave up diving because of a PADI medical exclusion, sold his gear, and subsequently returned to diving (when, as it happens, DAN & the diving medicine community concurrently decided that their previous positions may have been in error), I would offer the following thoughts:
1. I am happy to learn that you have had a great response to date to the surgery and medications. I do hope that things only continue to progress in this positive direction.
2. Though time seems to drag on at a nudibrach's pace (sorry about the joke, but sometimes a little levity is important) when you can't dive, one year post is not a long time in the neurosurgery and seizure business. You are to be commended for being so patient and reasonable, but please don't be surprised if your doctor suggests giving it more time before evaluating future risks associated with diving.
3. Assuming that you do remain seizure-free and otherwise asymptomatic for a good long time, a key consideration re diving will be whether the surgery is what has kept you seizure free or if it is the Dilantin. That is something you and your neurologist and/or neurosurgeon will have to determine. Sometimes, after extended periods of being seizure-free, a physician may start to evaluate with lower doses or "drug holidays" to see what's doing. Timing for such options? That's your neurologist's or neurosurgeon's call, since he/she is the one who knows what came out from where.
4. Other considerations may include the nature and location of the tumor, along with the functions controlled by that area of the brain. In addition, consideration needs to be given to the possibility of continuing, but more subtle effects from the tumor or the surgery. Some of our patients who have had a mild brain injury will seem to be doing okay, but over time, the patient, his/her family, co-workers, or friends will notice that some functional areas "seem a little bit off." In these cases, a neuropsychological evaluation (i.e., aimed at neuro issues impacting functioning, rather than psychiatric symptoms) may identify things that other methods have not.
5. Again assuming that everything is going great, who should you work with re evaluating the risks associated with a return to diving given your medical history and your current condition (notice the words - not "is it okay to dive?" Rather "what are the specific risk issues that your particular constellation of history and current functioning represent?"). In my opinion, this gets a little bit more complicated than some might suggest (and you are saying to yourself, "you mean it wasn't complicated before??!?") A neurologist/neurosurgeon with no expertise in diving medicine will be (WARNING: another inexcusably cheap aquatic reference is about to happen) like "a fish out of water" trying to consider what diving-related issues will be critical for you. On the other hand, many diving docs are more focused on pulmonary medicine and, knowledgeable as they may indeed be, won't have sufficient expertise to speak to the neurological nuances that may be important. (At this point, you may be hoping to find a one-armed physician who therefore cannot say "on the other hand" - sorry, I like bad jokes, what can I say?) If it is at all possible, you may need to bring together experts from the two disciplines. And please don't assume that an MRI or an EEG is going to give a definitive answer. These tools can only help guide the assessment process and the ensuing discussion.
Hope this helps. You may be saying to yourself, "hey, wait a minute, he never said whether I should 'hope or sell.'" Right! Because that decision, just like the matter of when or how a decision is to be made, must be between you and your medical specialists. Is this just one health care professional blindly promoting or standing up for another? Scarcely - as I alluded to when I started this reply, I gave up diving (and plans to move into the dive business) for 10 years because of narrow-minded medical thinking. And I'm not sure your deicison has to be an "either/or" decision. Faced with the "hope or sell" decision, I chose both, primarily as a means to make peace with my initial decision to suspend diving activities. It also forced me to be very systematic and deliberate in my steps when I finally decided to re-scale the issue of diving.
Good luck!
(and now the usual disclaimers - none of the above is intended to serve as medical consultation or in place of your seeking medical consultation or treatment. Only a physician who has personally evaluated or treated you can provide you with the guidance that you need. The information provided above merely documents issues associated with the treatment of certain medical conditions, in the hope that you will be able to more effectively participate in your own treatment and consultation with such physician)