You are right, the grand parent post was not absolutely correct. I also suspect that you already know the answers to your own questions. :) But in case someone don't:
"Can solar sails be used to reduce or change orbits?"
Yes! It's a bit overkill, but yes that is something one can do.
"Magnetic fields?"
There are very promising results in that direction too. A satellite can unfurl a long thin conductive tether. The magnetic field of the planet generates a current in the tether and a net drag on the sat.(To be 100% honest I don't understand this solution enough, but "Terminator Tether" is a term one can search for more info.)
"Physical characteristics and profile of sat vs orbit?"
Maybe! On low orbits you can count on the very thin atmosphere to provide you some drag. It's a common practice for example with cube sats to not put them too high, so even if something goes wrong with their electronics they will deorbit on their own in a few months or years. You can also increase this drag by turning your sat to "face the wind" with the largest available cross-section. This trick doesn't really work on higher orbits though, and that's where "space junk" is generally a problem.
"Put in a decaying orbit that is only maintained by thrusters?"
I'm not sure what you mean. Thrusters are rockets. But yes, that absolutely works. With the usual caveats. You have to budget enough fuel for the manoeuvre, and if your ageing sat craps out you might not be able to command it anymore.
I'm curious about these absolute claims. Can a heavy mass object moving more slowly entangle a faster object to reduce velocity.
Can solar sails be used to reduce or change orbits?
Magnetic fields?
Physical characteristics and profile of sat vs orbit?
Put in a decaying orbit that is only maintained by a thrusters?
Very interesting to hear that the only way to slow something down is with rockets. Not atmo drag. No mag drag. No solar pressure etc.