With the "continue" at Astro going nowhere - I hope there will be another WB 12 month quest line starting next month. Be kinda a waste to repeat cloak quest for those that have one?
I don't think it is a waste in the slightest. The cloaks have different stats/roles/uses, and also looks.
And if its still possible to obtain legendary cloaks If one has missed portent kills?
Well, if you read the patcher info every time you start the game, you'll have the answer to your question. Yes, as long as you kill 12 bosses, it doesn't matter which ones you miss. It'll take you longer to get the cloak, that's all.