Glad to hear iTunes does not have any problems with your ALAC & AIFF purchases from HDTracks.
Right, it's no big deal. If you opt not to transcode, 24 bit files at 44.1 and (I believe) 48 will go onto your iPhone as well. I personally have iTunes transcode everything to 256kbps AAC for my iPhone so I can a) get more on there; and b) not worry about maintaining separate copies of my higher res materials. It's a tradeoff that maximizes my on-the-go convenience, which works for me knowing I'm not going to be listening critically while I'm half-asleep on the train anyway.
I have one remaining question for you, do you happen to know why HDTracks states on their website "If getting the very best sound is important to you, you'll want to choose FLAC audio files for your purchase. ?
I tried asking them, they said (after a bunch of off-topic stuff) because "...FLAC has more depth to the sound in a proper hi-res setup."
Honestly, I felt their answer was not as clear (since they spent one line out of the 40 they wrote back to answer this) and they stated to choose whichever format would make my life easier (AIFF or ALAC for iTunes in this case).
HDTracks customer support is the pits. The few times I've dealt with them, their responses have ranged from vague non-answers to approaching beratement. Compare to the fine folks at, say, eClassical, who ask for customer opinions and take the time to write thoughtful responses... Dealing with HDTracks is... unpleasant in most regards.
As was mentioned above, if there's any difference in the 'quality' of lossless, it would be in how capable a processor is in decoding it. A computer these days shouldn't really choke on either. While there is software to play FLAC on the iPhone, I'd be really surprised if it managed to decode it as effectively as the inbuilt ALAC decoder.
Since the data is, in fact, lossless, and issues should only really present themselves when decoding on-the-fly, buying in one format and converting to another (say, for sites that only offer FLAC), should be a non-issue.
Glad to hear 192Khz wont be a problem and that iTunes is 32-bit ready (although in the sound playback options I can only choose between 16 and 24-bits).
If your DAC presented itself as accepting 32 bit streams, then the option should show up. Otherwise, iTunes will accept the files and the system will dither down accordingly.