Q1) Is this right?

Maybe. Most pressure reducing valves have have a bypass to allow the
pressure to equalize (if it is not clogged up). Check yours.

I keep my water tank at about 125 - 130. It has a pressure releaf valve on
it. I'm of the opinion that most people feel the probablility of needing
an
expansion tank is pretty remote. Give these conditions...

Q2) Would you think they are right?

I don't have an expansion tank because I turned my water heater way down to
let it cool off, and then turned it up extra high and monitored the change
in pressure. It never went high enough to matter, so I didn't bother with a
tank.
The conventional wisdom is that you need one because a) your t&p valve might
not work, and b) you don't want 120 pounds of pressure elsewhere in your
house. It is probably prudent to install one.

75psi isn't all that high but it is still better to use the valve and
possibly a tank. If your main valve actually shuts off, it is not that much
work to put it in. (I also put in an additional main shutoff as long as it
was opened up; the original one is probably pretty marginal.)