Jax,
Yes the sphincter pressure is relaxed with LIS and at its weakest in the first few months following surgery. It does strengthen up over the course of the year however not to the stage that it was before surgery. What this means though is the chances of a recurrence/re-tear are highly unlikely.
Diltiazem is a channel blocker which temporarily relaxes the sphincter but not to the same degree as surgery. Also between applications its effectiveness can wear off allowing the sphincter to spasm again.
If your symptoms are improving with diliazem then I would stick with it unless your quality of life is on the decline and you feel you have exhausted all of your options. Your diet is ideal but be careful not to overdo it on the fibre or your stools may start to bulk up and you may experience a lot of gas!!!
With regards to your fear of LIS this is only natural but you have nothing to fear – it really is a stroll in the park and my recovery was a breeze (read my daily updates above)
Unfortunately the incision will be the same size regardless of your fissure as this is just the entry point for the CRS to access your sphincter.
Davo