Do you need advance tickets for Arc de Triomphe

Just show up: we had the Paris Museum Pass but lines were not long there.

Yes, they do have a lift that's supposed to be only for people with disabilities but people without impediments report having been able to use it.

Using the Arc as a 2nd choice to the Eiffel is good idea as you never know what sort of weather you're going to end up with. Even if you'd been able to reserve the sunset hour at the tower, it's no guarantee that it won't be cloudy or raining. Assuming you have multiple nights in Paris, you can choose one for the Arc that's dry and clear versus being locked into a specific day/time.

If you have a Museum Pass, be sure to go up the Arc on a day when the pass is active, since it's covered. I used my pass to go up twice - once by day and once by night - and was very glad I did. The climb was well worth it!

There is a moving ceremony at the Arc de Triomphe every evening at 18:30. The flame at the tomb of the unknown soldier is relit by veteran soldiers or dignitaries. I once was present to see Nicolas Sarkozy do the honors. You may want to time your visit around that.

Actually I preferred seeing the lights of the city come on from the top of the Arc than I did from the ET. The added advantage is that from the Arc you can see the ET twinkle on the hour, that was great. Also it was fun watching the street lights come on on all the avenues that spoke out from the Arc de Triomphe, including the Champs Elysees. There was no line at all when I went there on a night in August - probably went up about 9:00 pm.