CDG timing
 

When should we arrive at CDG for a 1:50pm AF flight to JFK on Wednesday? We're in business, no checked luggage, US passports and hope to check in online.

What time should we get a taxi from near the Louvre to go to the airport? Or just monitor traffic with Google maps and leave with a sufficient amount of time to get there?

Goldorak gives a risk averse and wise answer.
With no traffic, it takes 35min from Le Louvre to CDG. With reasonable traffic at 11am, one hour is safe. Of course, there is always the risk of unexpected accident on the road, but, personally I would never leave before 11am. You will get at CDG before noon which leaves you plenty of time (almost two hours) to go through immigration and security.
Be aware that immigration with US passport can take long time (you have Skypriorty).

Is there any reason to prefer a taxi compared to an Uber (or Taxify) to the airport? If I understand correctly, taxi would be a flat 50 Euro, even if the hotel summons it. Our driver from the airport tried to charge 55 and his credit card machine claimed my card didn't work (it worked at the hotel 5 min later), which makes me a bit hesitant about taxis. Uber has worked fine, other than they always take longer to arrive than the app claims.

Any thoughts on Taxify? I have a 10 Euro credit, but that's not worthwhile if they aren't reliable.

No idea about taxify.
For airport transfer, I use LeCab as they are reliable and comfortable (Uber within Paris).
Note that this is school holiday, so traffic is likely to be lighter.

Le Cab does not appear to let you specify a destination and therefore does not fix a price. Am I missing something?

I'd rather have a price in advance.

g7, the leading taxi company uses the official fixed price scheme. I never had a problem with them.

Lecab does let you enter your destination and gives you a price. G7 is indeed good.
Frankly, I would not worry so much about 5 euro difference.

Do you enter the destination for LeCab after pushing book now?

5 Euro isn't a worry. Uber has had a range of about 30 Euros when I've checked (40s to 70s), which is more of a concern. With a fixed price, the driver's incentive is the most efficient route. Without, they make more money with a longer ride.

Use Snapcar.

Now our hotel is saying tomorrow is the SIAL Exposition in Villepinte, which they say will cause very strong difficulties with traffic and they are highly recommending public transport.

Looking at a map, it's not clear to me why that would cause a problem in Paris or why there wouldn't be a route from the Louvre neighborhood to CDG that avoids it

??

It seems hotels are always extra cautious about the time it takes to get to and through airports. At least, that's my experience.

Thanks to you (and others) for all the help in this thread.

Off to Terminal 2E in a few hours. We may take Uber, just out of familiarity, unless it has excessive wait times or charges.