Bread – What temperature of water will kill yeast

First off, it's good to understand the difference between active dry yeast and rapid rise yeast. Active dry yeast is a larger granule of yeast in which the outer shell is composed of mostly dead cells entombing the dormant nougaty goodness inside. It has to be proofed to seperate out all the cells and rehydrate the interior active cells.

Rapid rise yeast is a typically a combination of 2 different strains of yeast (so those with a very discerning palate might be able to tell the difference, I've never really tried). The granules of yeast are smaller and dried slower to preserve more yeast cells. Ascorbic acid is also added to push the yeast into overdrive a little faster.

A great video that helped me a lot is http://how2heroes.com/videos/dessert-and-baked-goods/yeast-101 I have noticed before issues with Rapid Rise yeast not proofing very well the second time around. Apparently it's designed to give you one really good proof and then die out (play hard and die young). Instant yeast however is Rapid Rise yeast without the hardcore party attitude. It's what they use at the King Arthur bakery test kitchen, so that's good enough for me! They say to use them interchangeably, but many other sites I see show a difference of about 20%.

2 Tbsp Active Dry yeast ≈ 1.6 Tbsp Instant Yeast

No, it isn't actually necessary. The yeast activates just fine with the moisture in the bread. I've been using Active Dry Yeast for years and hardly ever bother with proofing it.

Proofing shows that the yeast is actually alive. If you have any doubt about it, proof it as the first thing that you do, before mixing up the other ingredients (and especially before putting liquid in). If it fails then you didn't waste materials.