There probably is a lack of refining capacity for heavier crudes, and supplies consists of an ever growing part of these.

I really do think we are presently at Peak Oil, and this may be reflected in the refinery market where PO has been anticipated by not expanding capacity.

If the above is true, your analysis raises another important question. How long will we be able to maintain the plateau, especially in the light of heavier crude entering the market?

Here is an indication that more and more heavier crude is in the market http://www.energybulletin.net/8949.html
If there was an ample supply of sour crude that was not being refined it would be offered for much less than $50.  It is not credible that the price would go down if sufficient sour crude refining capacity came on stream.  In Saudi Arabia sour crude comes from small fields that could be called leftovers.  There isn't a sour crude Ghawar.  In Russia, sour crude comes from old depleting fields, mainly in Tatarstan.  There is not a vast amount of sour crude available in contrast to totally degraded oil found in tar sands.
That's right.

The spread between light and heavy oils has been growing though. While I think this is primarily because of decreases in available light sweet crudes, refinery configuration and capacity do play a complementary role.

According to Matt Simmons, the Safaniyah offshore field in Saudi Arabia produces heavy crude, and that is their largest field after Ghawar.