LOS - loss of signal
LOF - loss of framing
These occur when the modem doesn't detect the appropriate bits in the inbound data stream, though I'm not sure exactly which bits these are right now. Certainly it is more to do with the control data that surrounds the blocks of user data, rather than problems in the user data itself.
RSUnCorr is the count of the RS blocks that have uncorrectable errors, which is subtly different from the number of errors themselves. An RS block is the smallest unit that the FEC process works on, and has size "N" (in the modem stats).
RSCorr is therefore the number of RS blocks that had correctable errors.
RS is just the count of RS blocks.
OHFErr is a count of the number of X blocks or frames (can't remember the name of X) where CRC checks fail, but these are considerably larger than the RS blocks (variable, but perhaps up to 100x). A single RS block that is Uncorr is always enough to cause an OHF frame to count an error, but there can be multiple faulty RS blocks within the same OHF frame. Therefore OHFErr is always <= RSUncorr.
PFE = Path Far End (CRC is calculated per path)
FECS - correct, but I've never seen it either.
Your ES, SES, and UAS description are correct (strictly it is a CRC error that triggers these, but an uncorrected RS error always leads to a CRC error), but not quite complete. The counts also increment if LOS and LOF happen, but they're not so important in the grand scheme of things.