You should remember that the FBI apparently was able to seize several thousand bitcoin when they got his laptop (although this also may have been the result of seizing the servers in iceland).
30k came from a SR server, 140k came from his laptop. By the way, it's interesting to compare these figures to the estimated volume of the SR mixer.... I have a feeling they didn't find all the BTC, the numbers seemed way off when I looked at it. If I was running a website like SR I sure as hell would have a ton of cash tidied away safely, perhaps a paper wallet or such....
Ross' excuse for the 140k on his laptop was that he was an early adopter and high-frequency bitcoin trader and he earned those Bitcoins from that. This is what his friends and family thought he did for a living too. I haven't seen the FBI come out and say "these coins came from the SR mixer" or prove in anyway they were involved in the SR. It's a plausible excuse, I wish the FBI would investigate this side of the case more but they probably won't seeing as they'll likely win that asset forfeiture so why would they care.
I would think that he would probably have the bitcoin in a brain wallet instead of on anything physical that could be seized. A paper wallet would provide very little incremental security above having a wallet on his computer to someone in Ross's position.
Do you have a source as to the estimated volume that went through the SR mixer? Or even any documentation as to how it worked? That is the one mixer that I have seen very little information on as to exactly how it worked (as opposed to how it is much more clear as to how mixers like bitcoin fog and bitmixer, and blockchain shared send all work.
Are you speculating that this is his excuse for having so much bitcoin, or is this something that is public? I would think that a taint analysis of the various addresses of the bitcoin that was found on his laptop would likely connect Ross to SR, as IMO it would be extremely unlikely that he would be able to successfully mix that much bitcoin if it was coming from the site (assuming that Ross and DPR are one and the same).