I just built a home movie theater PC, and ran into the same problem. Let me address your questions one at a time. First question:
When playing a BluRay disc on the PC using a BluRay player program, can it decode the multichannel (7.1) LPCM/ Dolby Digital Plus / Dolby TrueHD / DTS-HD / DTS-HDMA soundtracks in their HD formats (ie, without downmixing to Dolby Digital or DTS or PCM) and output the audio directly to the soundcard's 7.1 line-level analog outputs?
What you're asking for depends on the sound card for the actual output. For the decoding, a very powerful method is to simply use ffdshow as the main codec, and use another media player (e.g. Media Player Classic) to playback the content.
Currently, support for the new Blu-ray specific audio formats (as well as the older - but still Blu-ray - formats) has been added in Beta 6 (see here for the changelog and download link).
To set it up, go to the ffdshow audio decoder settings. Navigate to the last option, the "Mixer" panel. Enable it, and set up your speaker configuration (yours would be 7-channel, and check LFE to enable the subwoofer).
Then, it should be outputted from the proper outputs on the sound-card. If they're just analogue jacks, you will need cable splicers to change them from the little headphone jacks (and you will probably need to amplify the signal).
Now, to address the second question:
Is it possible to bitstream the the multichannel (7.1) LPCM/ Dolby Digital Plus / Dolby TrueHD / DTS-HD / DTS-HDMA soundtracks in their HD formats (ie, without downmixing to Dolby Digital or DTS or PCM) over the HDMI output to a receiver when using a BluRay player program?
Well, continuing on with the ffdshow audio configuration, yes. You first need to set the HDMI as your primary audio output in your operating system. Then, navigate to the "Output" setting in the ffdshow config (just under "Mixer").
From there, you can check which signals you would like to pass-through unfiltered (e.g. AC3, DTS), your supported output sampling format (usually 16-bit), and the supported output formats if you want ffdshow to decode the stream (e.g. if you had a reciever that didn't support AAC, you could decode it and encode it to AC3 on the fly) or for formats which aren't supported by many recievers (e.g. raw MP3 data).
You have to note that it is a Blu-ray requirement that specific audio codecs are present on a disc, and that it is optional to have the newer formats.