Can I reinsert a partially discharged 12v battery back into a solar battery bank?
Yes, it is a bad idea.

Manufactures of the battery usually prescribe the maximum initial charge current in their datasheet. A typical lead acid battery has maximum initial current 0.3 CA or 0.4 CA for cyclic charging, where C is the rated capacity of the battery. For example, a 4.5 Ah battery with a maximum charging current 0.4 CA can be charged with 1.8A. If the current above this level, the battery can be damaged.

In your case, we do not know how much the current will flow from the higher voltage battery to the lower voltage battery. The higher voltage battery would charge the lower voltage in an uncontrolled condition.

The simplest solution is you can add a resistor to limit the initial current. For example, if the lower voltage battery is 11 Volt and the higher voltage battery is 13.8 Volt, you can use 2 Ohm resistor / 5 Watt to limit the initial charging current to 1.4 A. After several minutes or hours, when the voltage difference is not too far (say maximum 0.1v), two or more batteries can be safely connected in parallel without resistor.