All the materials are work-in-progress, so I appreciate inputs.
"DarkSend Transfers are very difficult to trace, if not impossible" can imply that with the proper motivation and means the tranfers can be traced, why not : "DarkSend Transfers are impossible to trace" instead, can we say that now ?
That's hard to say - I would never make that kind of statement and it would likely hurt our credibility. The entire field of cryptography is based around the principle that you can make things hard but never make them impossible. It is widely held principle in the related field of computer security as well - and it makes people who know the field nervous if you make a statement like that - and tends to piss them off.
Even if it were impossible theoretically - in general - there would likely eventually be a side-channel attack that would allow someone - under certain conditions - to trace it - again, hurting the coin due to a naive statement of impossibility.