When I said that
"It is this increased static pressure which causes altimeters to under read when the temperature is above ISA, and to over read when temperatures are below ISA"
I meant exactly that.
Imagine that you are out in space and you can see the entire earth and its atmosphere. Now imagine that the air up to some selected elevation is all red and that above it is all blue. If we increase the temperature of the air it will expand, causing the whole atmosphere to become thicker. If we look again we will see that this expansion has caused the junction between the lower red air and the higher blue air to move upwards. This means that more of the air is now above our selected elevation. So the pressure at this elevation will be increased. But the only reason that this has happened is because the air expanded. This expansion has reduced the density of the air.
So at our selected elevation the increased temperature has decreased the density and increased the pressure.
When your studies move on to Altimetry you will find that they over-read in temperature colder than ISA and under-read in temperatures warmer than ISA. The reason for this is as explained above.
Your second question is deffective in that it has two correct options.
Your reference to total pressure being constant is also incorrect. In order to convert an ISA atmosphere into a warmer atmospher, we must add some heat energy. This extra energy will increase the total pressure.