Total noise in an amplifier with multiple noise sources
I'll only answer point (1). Noise powers add; so you can use an RMS sum to add noise voltages. (square each voltage, add, and take the root of the sum.
BUT...
all sources must be normalised to the same gain. So you can:

RMS add V1,V2, then multiply by gain A of TR1, then RMS add that and v3

scale V1,V2 by A, then RMS sum ... sqrt((A.V1)^2 + (A.V2)^2 + V3^2)

scale V3 by 1/A then RMS sum with V1,V2 if you need the input referred noise.