Everyone has their own priority issues that they might consider all important. And if a vote for another party forwards their preferred agenda then they should absolutely vote that way. Just be practical about whether that vote will actually do anything to advance your agenda, or whether its just throwing it away in a seat where your preferred candidate will never win. There's feck all point letting the perfect become the enemy of the good.
So sounds like you agree that your original point, that unless you vote Labour you’re not entitled to complain about the Tories, was a bit daft. It also assumes that only through parliament can any change be achieved. There’s plenty of other ways of shifting the agenda and policy debate, in fact often they can be more effective. It’s a deliberate tactic of the establishment to try and channel all pressure through parliament and suggest that’s the only way to achieve change, it shouldn’t be repeated by people on the left.