You cannot pin the Offensive Line's failures on Shawn Watson. Watson was the OC first and QB coach second. His involvement in the actual factual coaching of the offensive line was minimal - or, to put it another way, it was about the same as his involvement in the coaching of the RBs and TEs, both units that excelled the last three years. If you're going to pin the O Line's failure on Watson, then you also have to pin the other units' successes on him, too.
Reality says that Cotton is the single most responsible person for the performance of the O Line. This is a reality Bo cannot afford to ignore any longer, and if I'm reading the tea leaves correctly, he is addressing it with this move to the booth.
Exactly.
I assume that Watson was responsible for the blocking schemes for the plays he was running, but beyond that, the O line wasn't his baby, it was Cotton's.