The easy answer to your first question is "No". HDCD is a proprietary encoding process (license owned by Pacific Microsonics) and HDCDs, while playable on regular CD players, sound better on HDCD-enabled CD players - HDCD-encoding is not decodable by conventional CD players. The more complicated answer may be "Yes and No." There exist, apparently, HDCD-decoding amplifiers, but I do not know how they work (I have been under the impression that HDCD is essentially a DAC filtering algorithm). If they do exist, theoretically one would not need both an HDCD-decoding CD player and an HDCD amplifier/receiver.
Sony SACD players do not, to my knowledge (I own one), have HDCD-decoding capability.
Sony SACD players do not, to my knowledge (I own one), have HDCD-decoding capability.