I'm travelling from London to Washington Dulles to Orlando with United Airlines. We get to Dulles at 10:55am connecting flight at 12:45pm. Will 1 hour 50 minutes be enough time to collect our luggage, clear customs and immigration, then check our bags in and then get to our connecting flight at Dulles? Do we have to go through Customs and Immigration at Orlando as well?
There is a good chance that you will make the connection, otherwise United wouldn't have sold you this ticket. However, there is a non-trivial chance that you will miss it.
You can check customs wait times here: https://awt.cbp.gov/. At Dulles the average wait times fluctuate a lot with time of day but average wait times are mostly 30 minutes or less.
You can also check security wait times here https://www.dhs.gov/how-do-i/check-wait-times but I think you need to install an app for that.
So maybe you need 60 minutes to get out of plane, through immigration, collect bags and through custom. Another 20 minutes to get through security again and that leaves you with 15 minutes to get to they gate before boarding closes (15 minutes before departure).
No if anything goes wrong (delay, unusually long waits, bag misplaced, etc.) then you will miss the connection. Assuming that's a single ticket, United will rebook you for free on the next available flight and may give you food and hotel vouchers if needed.
You do not need to go through customs or immigration in Orlando.
It should not be a problem, but maybe you miss the connection.
I had also a short connection (but in Atlanta), and because of long immigration process (longer than usual, the statistics), I missed the second flight. No problem, you go to transfer desk, and they book you in a successive flight. Just take into account that on busy period, the next flight could already be overbooked. So annoyance could increase.
My tip: try to get a seat as much as forward as possible on your flight. Study the map of the airport and go quickly to immigration, avoid WC (go before landing). This add some stress, but it could reduce the waiting time of 10 minutes (or more). Consider also that often intercontinental flights arrive earlier then timetable (on timetable you do not have the average expected time, but a "normal worst case" (so using a bad Atlantic route).
For the last question: no, the second flight is a internal flight, so will not pass to immigration and custom offices on destination airport.