My mom in her earnest harvested her rhubarb early and sent me stalks that are indiscernible from celery. It tastes very tart which I like but has very faint "rhubarby" flavors. A couple applications I have in mind are a standard cobbler and making bitters by infusing them in vodka. Is it safe to eat (i.e. does red stalk indicate that oxalic acids are no longer present in the stalks?) and how can I intensify the rhubarby flavors without overwhelming anything with the acids?
There are many cultivars of rhubarb, just like there are for most other vegetables. Surprisingly, it seems some cultivars with the best flavor have less of the prized pink color. The color of any particular rhubarb stalk will depend on both the cultivar, and how it was grown.
You should treat it the same way, regardless of the color. So yes, green rhubarb is safe to eat, with the same caveats as apply to any rhubarb. Especially, don't eat the leaves.
In On Food and Cooking (2004 edition), Harold McGee indicates that rhubarb tends to be about 1.5-2.0% acid by weight (mostly oxalic acid), which makes it quite tart. This is an inherent aspect of the vegetable. The only way you really will reduce it is to use proportionately less of it in your recipe, by either reducing the absolute quantity of rhubarb, or increasing the quantity of other ingredients.
In pies and coblers, this is usually done by combining the the rhubarb with another fruit like strawberries, as well as a great deal of sugar.
Oxalic acid is soluble in water at about 14 grams / 100 mL, and even more soluble in ethanol at about 24 g / 100 mL, per Wikipedia. This means that anything you infuse into vodka is likely to be extremely tart, perhaps unpleasantly so. You will want to carefully control steeping times and or ratios. While I have never tried this, I would imagine the tartness would overwhelm whatever rhubarb flavor you might pick up. There might be a reason there are so few famous rhubarb cocktails.
Still, there are a number of rhubarb cocktail recipes that you can find by googling "rhubarb cocktail". They tend to start with a cooked down puree of rhubarb, rather than an infusion, however, and it seems to often be used in the sour role.
Indeed, the green stalks were delicious in cobbler and the infusion is coming along nicely. The flavors and textures were perfect and the color remained greenish. No kidney stones to speak of... yet.