Anyone have any long-term experiences regarding (wall) mounting
> rack mountable kit front-down or front-up? Reliability,
> accessibility, etc. And, any suggestions to protect the
> openings-susceptible-to-gravitational-interference?
If it is all on forced ventilation then the only problem might be that some fan bearings are only designed to be in one orientation.
Passive heatsinks won't work as well if their fins are not aligned with the direction that warm air wants to rise.
> Any suggestions as to which (-up or -down) might be better?
I prefer not to have to crawl under things to see the front panel YMMV. My pet hate are the input connectors on big display screens in positions where it is just about impossible to reach them to plug in a cable.
For a start, I'd like to mount six 48/52-port switches.
> Most of the mounting kits suggest this is within their
> typical weight limits. But, 4U seems to be the most common
> arrangement (I'd like to have to avoid using two kits)
Provided they don't run too hot should be OK.