Zurich Flight/Train Connection

Probably. I assume you will have already cleared Schengen immigration earlier. Also, unless you are getting a Super Saver Ticket, you can get a train with a connection at Weinfelden at 18:16, or the next direct train to Konstanz at 18:46. For that matter, you could even make the earlier connection at 17:16.

We have been advised that reception at our Konstanz hotel closes promptly at 20:00. I am concerned that we could not make the 17:16 train, therefore I wanted to plan on making the direct 17:46 train arriving Konstanz 18:50. Yes, there is an 18:16 train, with a connection, but it arrives Konstanz 19:34, which is cutting it a bit to close I think.

Possibly, but it doesn't matter. The possible problems are:
1) Where is your flight from? Could it be late landing?
2) Delays at immigration. Zürich is fast, 30 minutes should be OK.

But it doesn't matter, there are trains from Zürich airport to Konstanz at 16:46, 17:16, 17:23, 17:46, 18:16, 18:46, . . . .
You just walk across the road (undercover) to the station, buy tickets (CHF 30 per person) and get on the next train going in the correct direction. Tickets are valid on any train, simple!

Just pointing out you have options. If you buy a full fare ticket ahead of time, you can take any train that day from Zurich airport. If you land at 16:20, it should be no problem what so ever. But what is your back-up plan if your Lufthansa flight is delayed? That is something neither you or I will have any control over. If you are that worried about it, find another hotel.

Immigration is fast at ZRH....if you could make an estimate as I’ll be arriving from JFK, about how long would it take to go through immigration including the walk to passport control from the arriving aircraft?

The trains aren’t your problem...it’s the airlines. Just a couple of days ago my daughter was flying home to Geneva starting the trip from Tennessee. Little plane here had mechanical problems and was delayed 8 hours which of course made her miss all her connections. Bottom line is that she arrived back to Geneva 24 hours later than planned.

What kind of hotel wouldn't have a way to make keys available to late-arriving guests? To me this is the issue - not the trains, which are flexible in Switzerland (exc Supersaver fares).

"What kind of hotel wouldn't have a way to make keys available to late-arriving guests?"

Many smaller and less expensive accommodations, in many countries, don't have 24 hour reception, and don't always have a way around this if you arrive late.

For the OP, it's something to consider - should you change to a place that will be able to accommodate a later arrival, just in case?