We have a few here and haven't noticed any noises, but then my labs and our production areas aren't all that quiet, so a faint noise would not be audible unless asked everyone to turn everything off in one of the labs...
in the case of RAID5 and RAID6, there is only one stripe size used in an array. Both use block-level striping. (in practical context, one only says stripe size-- I have only ever seen it where block and stripe sizes were identical)
You may have different arrays with different stripe sizes on the same controller.
(nested RAID levels make this potentially more complicated, but the fundamentals remain the same)
Wikipedia's breakdown is pretty accessible:
RAID5/RAID6 operate in a user selectable on creation, fixed stripe size. I am not familiar enough with RAIDZ or RAIDZ2 to say with authority. I have very limited familiarity with PAR2 and cannot speak at all as to best practices for PAR2's concept of recovery blocks.
The change in transfer rate looks right, the further you get on a spinning disk the fewer sectors travel under the heads in a given time, so the transfer rate decreases. It's a function of where you are reading from on the disk, not of temperature (unless of course you are overheating the disk-- but 44C is within operating spec, so that shouldn't be a factor).
Phanteks Enthoo 719 just came out, I'd say it's one of the few that supports that many drives at a non-crazy price.
Looking at photos, the Corsair 900D might, but then you have power supply fitment limitations and a lack of cooling for some of the drives. Adapting the 5.25" bays to fit some 3.5" drives might alleviate the power supply fitment limitations...
If you swap the failing disk out with a known good one, does the RAID rebuild? If so you can be pretty sure the disk is bad, SMART data or not.
If you are concerned about warranty, 99.999% of manufacturers will accept a "drive will not stay in array" as a failure code.
It's number of drives in the array (n).
For RAID5, 1 drive's worth of storage is reserved for parity data, so (n-1) is the available capacity.
For RAID6, it's 2 drive's worth that is reserved for parity data.
RAIDZ and RAIDZ2 are similar in the space reserved for parity.