JRIver SACD ISO playback doe not require any DirectShow plugins, etc...it's native for DSD-capable DACs like I assume the DAC2 is. My Meitner and Mytek (both stereo and stacked for multichannel) play back ISOs fine. As you stated, it's likely an install issue or bug with DAC2 (not a JRiver general bug cuz I can play them fine). That being said, ISOs are a pain in the ass, have minimal tagging, usually sketchy cover art, and can be a cpu issue (like my minimalist CAPS) if DST compressed. I'd recommend, even for you demo listening, to extract to DSF.
As for ASIO vs WASAPI, there is no good answer..it is entirely DAC/driver specific. For my Meitner I cannot use ASIo, so I choose between WASAPI-Event Style and Kernel Streaming. For the Mytek I love its ASIO driver (although can do WASAPI-Event) and love that the ASIO driver allows DSD128 playback as well.