Pre-mixing liquor -- any downsides?
There should not be any problems with mixing alcohol as long as you don't introduce organics.  I would suggest mixing the alcohols and leaving the lemon juice and splenda out until you pour a drink.  Keeping alcohol cool and in a dark place.  As for indefinitely, no.  The amount of time it stays good is dependent on the amount of air in the bottle. Glass is always preferred if possible.