I think the fading in and out has to do with the duration of the grain.
The sample is always playing back using grains and they have a certain duration.
Even at the longest duration setting, it is still a grain (slice) that has a set length.
So it fades in and out at the beginning and end of the grain, no way around that.
Adding another grain stream with the right offset can hide the fades to some extent.That is my guess anyway…
I think you’re right here Rotund and I wonder how hard it would be to write some extra code to include a signal path (at the start of the chain) for the unaltered sample file to run into a mixer parallel with the altered version? And if we’re really dreaming here then an extra signal path for the unaltered sample to run into the filter with mod options like envelopes, lfo’s etc and the ability to then feedback this signal to the chain start 
But really would be nice Steinberg to at least allow a mix of unaltered and grain altered samples. Hope you’re listening 