The claim that 'the people who killed Nijjar were too young to be state actors' is fallacious to begin with. It's even more tenuous in light of everything else that's known about the murder.
It goes without saying that there's no reason to give any weight to India's denial of involvement. That's all that really needs to be said about that.
Canada expelled an Indian diplomat. There's no reason to do that if India wasn't involved. There's no reason for India not to cooperate with investigations if they're not involved. We know that Five Eyes intelligence exists that makes a connection between the assassination and the Indian government. The intelligence itself hasn't been disclosed (and never will be - sources & methods, etc). So, waiting for that kind of disclosure before forming an opinion on this is folly at best.
The current Canadian government is horribly weak on matters of foreign interference, so if they've been mealymouthed regarding this assassination, I don't think that casts doubt on India's involvement. If anything, it's a suggestion of the opposite.
Given the degraded state of Canada's current foreign policy, it's expected that they would tiptoe around confirming a direct link between India and the assassination, and may choose to never confirm it. That doesn't mean we should infer that a link doesn't exist.
With all that in mind, I don't see any reason to conclude that India wasn't involved.