In heavy-bottomed saucepan, heat rhubarb and sugar with 2 tablespoons water over low heat, covered, about 10 minutes or until rhubarb is soft, stirring occasionally.
Strain syrup through fine strainer or colander lined with cheesecloth. Refrigerate.
To make Orange Milk:
Remove zest (orange part of skin) from oranges with vegetable peeler.
In saucepan, heat milk and zest to just below boil; remove from heat and steep, covered, 1 hour. Strain; add sugar. Refrigerate.
To make Basil Syrup:
In saucepan, simmer sugar, corn syrup and vitamin C with 1/2 cup water until slightly thickened. Cool.
In blender, purée syrup with basil and pinch of salt; strain through fine strainer. Refrigerate.
Sweeten strawberries with sugar to taste.
For each serving, in chilled 16-ounce glass, layer 1/4 cup Rhubarb Syrup, 2 tablespoons diced strawberries, two 1/4-cup scoops ice cream, 1/2 cup Orange Milk, and 2 more tablespoons diced strawberries. Add 1/4 cup seltzer; drizzle with 2 tablespoons Basil Syrup. Garnish with 1 basil sprig.