My feeling is that the trout eat snails as a last resort when they can't find something better to eat. It has got to be difficult to pass those snail shells through the digestive tract, and it can't be very comfortable (like having a bunch of rocks in one's stomach). Rather than trying to match the hatch with a snail pattern, I recommend just trying one of your favorite go-to fly patterns, like your black Simi Seal leech, as the trout are opportunistic and would probably much rather take the leech than eat a snail. Even though it seems that the fish is keyed into just eating snails, it is more likely that the fish would be eating other things if it could find them. The fact that you caught the fish on a black leech when the stomach was full of snails proves the point.
Your experience also illustrates well the problem with throat sampling, in that larger items cannot fit into the pump tube. I gave up throat sampling many years ago just for this reason. I can see that when you're chironomid fishing, it may be helpful for matching the hatch, but most times I think it is less stressful for the fish to not throat sample as you're only getting a partial picture of what the fish has been eating anyway.
