Isnt that only if you decide to go FDCL?1. In theory, yes, it's a very minor DPS loss to use the 2nd death before MB assuming MB is off CD. In practice the difference it'll make will be nearly negligible.
2. If you are casting the 2 Insta-MS while MB is off CD, yes it's a DPS loss. 2 GCD + Mind Blast cast time = ~2.7s, casting MB = ~1.15s (Haste level dependant).
In regards to 3. - It's a DPS gain to have MB on insta-cast compared to cast time. You save time and can continue the rest of your rotation faster than normal, which is why Glyph of MS is a DPS increase.