Does the pliatropy (I can’t spell) concern influence the second assumption as well (i.e. no SNP influences on confounders)? Thanks 
Replying to “Does the pliatropy (I can’t spell) concern influen…”:
it certainly does. A way to check is to do a phenome-wide association study on your SNP instrument to see what it is associated with, and exclude it if it’s associated with a potential confounder
Replying to “Does the pliatropy (I can’t spell) concern influen…”:
If the effect is of the SNP → confounder (pleiotropy), it would violate the third assumption and if the associaiton is confounder (e.g. pop strat) → SNP, the second assumption is violated!