I have never cooked frozen lobster but have cooked numerous fresh whole lobster and crawfish (not crayfish) over the years but all I have ever done is around 20 minutes per pound in boiling sea or well salted water. I have never grilled them but have boiled then onto a bar-b-q as most times mine were cooked outdoors. Lobster is blue/brown and will turn red when done, Crawfish is already an orangy/red so needs timing. Don't overcook as they could go rubbery. If as you say you are having tails only I would, if it were me, reduce the cooking time by 5 minutes per pound but please ensure if, as you say, your tails are pre frozen that you defrost them completely before cooking. I would want 24 hours in the fridge top shelf or cold larder at least.
I'm sure others will have all manner of fancy fish or garlic sauces to go with lobster, I like mine as nature intended but on occasion might just compromise with a little fresh lemon, and in years gone by I caught them and cooked them within a couple of hours. Enjoy them, my mouth is watering as I write. I remember in the late 60's on Lundy Island each Friday we ate Crawfish, Lobster, Scallops and flatfish all caught by us an hour or two before cooking. Happy days!
