Yep! See this thread for the continuation and updates: The Journey to One Petabyte - Chia Farming & Harvesting - Chia Forum
Nope! I just got tired of LARPing as an enterprise JBOD designer 
Agreed! I didn’t have any vibration issues but I do like the heavier racks and JBODs now that I have them (see updated thread)
If your goal is cheapest $/TB, smaller drives are way to go. If you have cheap electricity and plenty of connection points (SAS or SATA) then it makes sense. Any drive uses $7-10/year in electricity at 10-12c/kwh. For the smallest drives, you are paying maybe $7-8/TB if you buy in bulk or find good deals. But my target for cost per drive for connections (SAS JBODs in my case) is about $10. So if you just plan on running them for a few years and then upgrading, smaller drives might make sense over larger drives, especially if you already have the connections. If you are out of connection points, buying larger drives might make sense so you don’t have to buy as many more connections (SAS/HBA/JBOD/etc). 
YES! See more details in continuation thread above.
Yep! And growing. Check out me and Jerod (Digital Spaceport) unloading our latest auction win here:
Digital Spaceport Storage Wars - The Breakdown - YouTube
Digital Spaceport Server Wars Final Analysis - With Josh Painter (Endertown, Catmos) - YouTube
If you are scaling up to petabytes, you’ll want to stop buying retail (or even ebay) and start looking for local/state auction deals on lots of server equipment!
Hope that helps, good luck!