I think we are on the same page. Just FYI but Juniper HW support is determined by SN. I assume you purchased the 2nd power supply as redundant after the fact. For that to get service, someone (Partner?) needs to tie the PS to the EX chassis. That is not the case for Optics, so you should be all set there.
Yes for much of EX Access switches, PS and Fans are NOT FRUs - Field Replaceable Units. So for such failures you'll get a replacement switch, for which you take out the parts you need and then put them into the switch you received and then send back whole switch. Yes, may not make much sense, but way it is.
If you had an Optics failure, they will not send you a whole switch -