It's very common. In fact, there have been rabbits on WR pace in every recent successful WR attempt. London also isn't a particularly fast course, and it can get quite warm.
Farah has run a couple of 13.1m since he has been at NOP, and none of them have indicated he has close to WR pace. He has since been training specifically for the marathon, but so have many of his competitors.
A WR is mostly a time trial, and race tactics don't really come into it. But yes, Alberto is smart enough to know that Farah shouldn't attempt a WR.
It seems much more likely that his real talent is between 1500m and 5000m, compared to all the other running talents he'll be up against in London (including the current WR holder).
15:1 for the win seems about right - but that would largely depend on the possible but unlikely scenario that about five of the other contenders go for the WR and all blow up, while Farah sticks to a more sensible plan, and comes back to win in 2:05-2:07.
Farah will not run a WR, and if he goes for it, it reduces his chance of winning. The chance of Farah running a WR is far less than 2%