Could transmit/receive turnaround be improved by having a heterodyne receiver switchable between high-side and low-side injection
When a transceiver is transmitting the PLL frequency is set to Fs and when it is receiving the PLL frequency is set to either a frequency greater than Fs by the intermediate frequency  or one lower than Fs by the intermediate frequency i.e

When transmitting:

$$
F_{\text{PLL}} = F_{\text{S}}  
$$

and when receiving:

$$
F_{\text{PLL}} = \begin{cases} F_{S} - F_{IF} &amp; \text{for low side Injection} \\
    F_{S} + F_{IF}  &amp; \text{for high side Injection}\end{cases}
$$

Remember that we are only permitted to transmit over a very low frequency range e.g 2.39GHz to 2.42GHz so we can not use any of the approaches you propose above because Fs has to be constant.

Further notice that for use the only way to avoid retuning the PLL is to set the intermediate frequency to zero, which will essentially now turn our superhet receiver to a one stage TRF receiver (You can read the the disadvantages of TRF receivers in the link provided).

As for the solution you propose for reducing the IF frequency to a very low frequency so that we can reduce the amount of tuning to be done, you have to remember that the lower the value of our IF frequency the poorer the image channel rejection our system will have so there is a limit to how low our IF can be.We can't just set IF to a very low value.