MOSFET burned and high heated bed resistance
The resistance of the heated bed being too high can not have caused the MOSFET to burn out. Only a too low resistance could cause that.
Keep in mind that measuring relatively low resistances (such as the one of your heated bed) is difficult, and if you just used a regular multimeter it might indicate a wrong value (e.g. due to the resistance of the test leads or a poor connection between the probe and heated bed). Therefore, the actual resistance might be (slightly) lower.
A 1.8 Ω heated bed at 12 V will draw around 7 A of current. This is well within the capabilities of the MOSFET on board of the RAMPS. So either the resistance of the heated bed is lower than you measured (if the resistance was 1.2 Ω or lower this could cause the MOSFET to burn out), or you simply got unlucky with the quality of your RAMPS board.
The (supposedly) high resistance definitely doesn't make the heated bed compatible with 24 V. Using a 1.8 Ω bed with 24 V would cause a 13 A current draw and 320 W of power. This is a lot more than is sensible for a 214 mm x 214 mm bed.
All of the alternatives 1 and 4 you have listed are reasonable options. There is no reason (option 3) to replace the heated bed as there is no indication it is faulty. I see no reason to take option 2 since it involves desoldering and resoldering the fan MOSFET for no reason (if you are using an external MOSFET the fuse is no longer a limitation). There is a chance you'll damage the MOSFET doing this and option 1 only requires a trivial firmware change.
Option 4 (upgrading to RAMPS 1.6) is purely a matter of personal preference.