I think you're not getting any response because you posted this question in the wrong forum. There's a very active rhubarb thread in the Beginners Forum.
I believe the "rule" about picking rhubarb before July 1 comes from the quality of the stalks declining over time and with higher temps. I don't believe there is anything wrong/contaminated/poisonous or other problem with using them for wine. There's no doubt that during midsummer the quality of the stalks is not the best for baking, but there won't be any problem using them for wine. Just don't use the leaves of course.
I do several harvests over the spring & summer, always leaving at least 3 stalks with leaves, then fertilize immediately. It always sprouts right away. Rhubarb is a water hog and really likes extra nutrients too.