Shunt ohmmeter question
I agree with you that when at 0 (zero) Ohms the meter is at full scale (37.5 uA) it must be at half of the full scale when \$R_x\$ is 24 ohms as that halves the current \$I_b\$ so must also halve the current \$I_m\$.

So for this circuit 24 ohms is half-scale.

Now let's look at an analog multimeter with a R x 1 scale:



And another one:



Note how for the top one 5 ohms is at half-scale.

While for the bottom one 10 ohms is at half scale when using the R x 1 range.

Obviously it depends on the circuit inside the meter what value corresponds to half scale. For the circuit from your question half scale would be 24 ohms.

Also notice how high ohm values are at the left (small) and 0 ohms is at the right. This also means that the maximum value of a an ohms range is infinity.

Your professor stating that the scale has a maximum of 10 ohms is simply wrong. The maximum is at 0 ohms as then \$I_b\$ and thus \$I_m\$ will be at maximum.