I think with Bluray a lossy track is certainly mandatory, but whether it is Dolby, DTS or both is up the content provider. Fox's discs for example don't have Dolby at all, just DTS HD Master Audio which has lossy 1.5mbps DTS stream imbedded within it.

I'm not aware of Bluray players downmixing LPCM to DD 5.1. To the best of my knowledge if selecting LPCM on a Sony or Disney disc it'll only downconvert it to 2.0 PCM over coax/toslink.