What can I do with super runny icing?
I'm going to ignore the actual icing ingredients for a moment here as I suspect they will be mostly icing sugar and a few neglible other things.

So you now have roughly 


1 cup / 250 ml milk
1.5 eggs
6 tblsp / 60g starch and flour
a lot of icing sugar


-> The egg means you need a heating step to get that mixture safe.

If we ignore the sugar, we are looking at a basic pudding mix, albeit with somewhat skewed proportions. So some minor adjustments are in order:


Add another 1-21 cups of milk, stir well.


Heat slowly on the stove, stirring constantly until the mixture thickens.
You'll get some version of pastry cream / pudding. It will probably be somewhat floury and very, very sweet, but at least safe and basically edible. 

Once you have the pudding, you can use it as a base for desserts, cake fillings etc., mixing with fruit or stretching it with yogurt could tone down the excessive sweetness.

You can also use the thick pudding for a custard-based buttercream, which means you finally turn your mishap into a frosting again.



1  The math, very rough numbers as I don't have the exact amounts you used:

1 tbsp starch / flour ~ 10 g

For pudding, you'd use 20 g / 2 tbsp per 250 ml / 1 cup.

So if your guestimate is correct, use 3 cups milk in total.  

For a somewhat thicker pudding, use 30g / 3 tbsp per 250 ml / 1 cup.