We're going to Bonn May 16 2018, RDU-LHR-DUS then taking the train to Bonn. Tickets are from AA with the last leg on BA. I'm AA Plat/OW Sapphire. Neither of us speak German.
Questions:
Should we buy train tickets in advance, or buy them there, and if the latter, should we have EUs in hand?
Time frame for clearing customs, immigration, and getting one checked bag: an hour? More?
Our departure (DUS-LHR-RDU) time on Monday, May 21 is 7:20 am. We plan to stay at an airport hotel the night before. Is getting there by 5:20 am early enough to check a bag and get through security?
Will two metal hip replacements mean extra time going through security?
Does OW Sapphire status speed check-in up, or no?

Sorry for the ignorant questions--I'm a FF within the US but this is the first European trip, and the reviews of DUS have been a bit daunting to where I'm wondering if we should come into FRA instead.

You will need around an hour to clear immigration, customer and to get you luggage. With some time buffer you should take a train about 1,5 - 2 hours alert your arrival.

The easiest way is to buy the train tickets in advanced and have your itinerary printed out. Just remember there are different types of tickets available from Deutsche Bahn. The cheapest tickets don't give you any flexibility. You can only take the exact train that is printed on the ticket. The more expansive tickets give you the flexibility to take earlier or later trains on the same day. Easiest way to get form D�sseldorf to Bonn by train is the Regional Express train which takes an hour to Bonn without changing the train. Just book your tickets https://www.bahn.com/en/view/index.shtml . Search f�r D�sseldorf Flughafen - Bonn Hbf . Be aware D�sseldorf Airport has two train stations. One is D�sseldorf Airport Terminal right under main building the other one is just D�sseldorf Airport a bit outside of the terminal. I recommend to take a train from D�sseldorf Airport and not Airport Terminal. You need to take the terminal shuttle train to get there. The shuttle will take around 5 minutes to get you to the train station.

Yes, you should have EUR with you. While you can use your credit card for a lot you will also need cash at many places, especially restaurants tend to prefer cash or just debit cards but not credit cards. There are exchange booths at DUS airport.

I thought metal implements are non-magnetic and do not trigger the metal detector? If they do you would need some kind of medical notice form your doctor to get through security.

It does not matter if you fly to DUS or FRA.

In DUS they don�t use metal detectors any more. They have body scanners. Regarding the replacement in doubt they do a manual check and it should be fine.

Your flight fro LHR will arrive at terminal B - usually at gate B 37 or close by (though that could change till May 2018).
Immigratiion is done at the gate and if your are in the front wait should not be long. Waiting time for luggage could be a quick or a long wait, I experienced everything from 5 minutes to 20 minutes. But overall I would say play safe and at least on hour after arriving.

Regarding train ticket, it depends if you prefer a regional train or a long distance train. Not all long distance trains stop at D�sseldorf Flughafen Fernbahnhof, in so far taking the S-Bahn to D�sseldorf train station and change there to a train would be an option. Given that there might be some schedule changes, I would wait with buying train tickets or buy them flexible on bahn.de. A ticket for ICE entitles you to use the regional trains as well.

5:20 in the morning is okay. If your flight leaves at 7:20, boarding would start around 6:50 am (I am usually there between 05:45 and 06:00 with checked luggage). But being earl(ier) thatn 06:00 could help with security.
BA uses the Hugo-Junkers-Lounge, which is open from 05:15 am, so you gave a place to spend the time till boarding. There is a fast lane, but as written in other posts, nobody checks it and all ticket holders can basically use the fast lane ...

Metal hip should not be a problem, just let the security people know (they use bodyscanners).

Those time frames are just what I needed--thanks! And the explanation of the different train stations really helps. Thanks for the heads-up about EUR and CCs as well. We do prefer smaller restaurants so I'll keep in mind cash is the way to go.
The metal is non-magnetic but still trigger the metal detectors in the US and JA so he gets wanded every time even with TSA in the US. He does carry a card re: the implants.

Thanks again. This is very helpful.

That is perfect info re: unloading times and morning flight times.

Good to know about the Hugo-Junkers-Lounge. I've got AC membership and Priority Pass and it looks like PP gets me in (although might have to pay for the 2nd person--more info to check).

Thanks!

OW Sapphire gets you and one guest in the lounge if flying oneworld (=BA) ... no need to fork out the PP

As for train tickets, it would be cheaper if you buy them in advance, even if just one week in advance. However the cheap fare would involve a penalty should you need to change or cancel and you need to do it a day in advance.

It should be noted that train tickets departing from DUS should actually always be purchased in advance, for two probably not too obvious reasons:

First of all, technically, you will need to be in possession of a valid ticket for the people mover between the terminal and the long-distance railway station (although my ticket has never been checked on really numerous rides), and you cannot really get a proper ticket at the airport (they only sell tickets for local transit, which I think would be too complicated for any visitor).

And then, there are only 4 ticket machines at the airport station, which are quite circumstantial to use (even if you use them on a daily basis) and are hence usually occupied by desperate, newly arrived people who don‘t really know how to use them (and there‘s nobody there to help then). It‘s actually quite likely that you‘ll miss your train due to the wait at the ticket machines.

So, always buy your ticket in advance when traveling via DUS.