The demonstrator going to Mars on the 2020 mission can only fly for 90 seconds per day and afaik has no science objectives other than trying to fly. But if it proves viable they could presumably scale it up to something with a longer range (say 5-10km/day).

Correct, it is going to test the technology and take photos with its 2 color cameras.

This is basically a project that was "Let's try this, how much weight does the mission have to spare? Ouch, that's not a lot, let's see what we can do", that's my takeaway from this episode anyway https://www.planetary.org/multimedia/planetary-radio/show/20...

Could a helium balloon fly in the rarefied Martian atmosphere?

Here's NASA's page on the subject https://mars.nasa.gov/technology/balloons/index.html

Yep. I actually ran the numbers on this (in a spreadsheet) and even ordered a weather balloon to try and built a DIY survey balloon that might also work on Mars. Got bogged down by the hardware side though, and abandoned the project. :D