+++I wonder why Merck would choose to enter the race so late in this way, given the problems associated with monthly intravitreal injections.+++
This is a low-risk partnership for them. 20 mil is not a lot of money for them (and the full amount is not even guaranteed.) Their real expenditures will come if they choose to develop it, and they will only go with it if it looks to be successful. I'm sure that part of that decision will be an assessment of the competitive landscape at that time. Small downside, potential for good upside.
Besides, who knows if they are not thinking of them as a potential acquisition down the road, and this deal just brings them closer.