Of course a net positive is a good thing. But so long as there is a difference between that which is best for a given person and that which is best for the planet on which we rely, there will be an ethical question. In other words, so long as "solar-lacto" is worse for a person but better for the environment and "solar-meat" is better for a person and worse for the environment, the question will remain.
It seems reasonably clear that a doctor's responsibility would be to recommend only that which most promotes the health of his/her patient. Similarly, an environmental scientist would be bound to recommend that course which offers the best possibility for long-term environmental health. The predicament falls on those who are concerned about both.
I am not arguing for any one answer to that question. I just thought of it recently and was interested in what other people thought.