I didn't know they had a "garden" tour also! I, too, am interested to know about this. Can anyone help us out!?
But, I think you should be able to do both. The only issue would be how far of a walk it is from St. Peters where the SCAVI ends and where the entrance to the garden is. I wouldn't worry.
Also, as to time frame for SCAVI confirmation. I emailed in March for 3rd week in Sept. No response (no auto-reply either). I emailed again in June (and got an auto-reply). I just heard last week that I have a reservation for 9/22. So, they should be starting to fill the September dates now. I'd re-send a request (especially if you didn't get an auto-reply). Good luck!