Framer, you ask excellent questions. Your PA along with the actions of the cabin crew will define the passengers perception of the professionalism of the whole situation. If a journalist afterwards interviews passengers and they all say they felt that they got insufficient information and the cabin crew seemed like they didn't know what to do it may affect the public image of you company afterwards as well.
Some suggestions:
In the case of a "normal" engine failure during departure and an immediate return for a flaps 15 landing to a long runway, you are not doing an emergency landing.
For that reason the cabin crew will not give any instructions to the passengers.
(Hence, the the cabin crew don't need a NITS briefing either). Nobody have to do anything at all, except remain seated and fastned.
Luckily, that is what they are all already doing!
So the only purpose of the PA is to avoid anyone panicking. The way to avoid that is to tell people what to expect.
With your PA, the passengers will expect the cabin crew to perhaps prepare them for an emergency landing and during the 15 min where they don't get any instructions they may get worried and some may panic. For sure they will expect some kind of information during the 15 min, because you promised them that. Add to that the burning smell and slight smoke that comes creeping throughout the A/C. Since the cabin crew is not doing anything some passengers may perceive that they are "struck with panic", at least that it the interpretation the journalist will write.
So a better option would be to tell them just what to expect:
"Ladies and gentlemen and cabin crew, please pay attention, this is your captain. We had a problem with our left engine and we have chosen to shut it down. For safety reasons we will return to XYZ for a
completely normal landing. If you smell or see a little smoke in the cabin, this is normal and comes from the engine through the aircondition and is harmless. We will be busy the next 15 mins to complete our checklists and set up for the landing, for that reason further information when we are on the ground. At this time I ask everyone to remain seated with you seat belts fastened and await the landing. Thank you."
Speak unusually slow and make a pause between each sentence. Since your adrenalin probably is a bit up it will ensure that you actually speak at a normal pace.
Followed by an intercom ping and asking the Purser if she copied that and if she has any questions and telling her and the girls to remain seated as well.
Some may disagree with the NITS thing, since they want to see you perform it in the simulator, but in this scenario it's BS. You are not preparing for an emergency landing.