I was just in Bavaria two weeks ago. Radler is definitely not a mix-in of 7up/sprite. Traditionally it's lemonade but most places do use a lemon fanta equivalent these days.
+1 to those who said to just brew the Helles and mix it in separately. That way not only do you avoid having to argue the above ("you added the wrong thing!" - just let people mix in what they want), but you also dodge many possible complications.
More importantly, I think the way that larger brewers are able to pre-bottle these is by pasteurizing / sterilizing the beer to halt any additional carbonation and then by force-carbing the bottles. Don't expect the same results at home through bottle fermentation!
Also, I find that the pre-bottled versions such as Leinenkugel's Summer Shandy pale in comparison to the real thing and taste very "chemically" and not at all what it tastes like if you get the same on tap and mixed in fresh. I suspect that they make some trade-off modifications for shelf stability.
There's too many moving pieces here and you'd be trying to re-create something that isn't really a small batch bottling operation. Maybe there is a path to be found to good results but it would be a roll of the dice. Brewing and bottling a Helles on the other hand, is predictable!