Hi Rachel! Since lying tricep extensions target the triceps more specifically than close grip bench presses, I would rather you do an exercise that also targets triceps more specifically. You could do the cross body kickbacks that we did in Pure Strength, or seated (or standing) one arm overhead extensions, or one arm tricep kickbacks. To substitute the side lunges, you could first try to modify the move by doing the exercise the exact same way except you would side lunge onto a step so the drop is not as far (do all on one side first, then switch sides). If that still is uncomfortable then do alternating back lunges. While these will not target the inner thighs as much, you will recruit some inner thigh muscles. For posterior delt, you can stand bent forward with your legs in a forward back split stance. Put a dumbbell in each hand with the thick ends facing the side walls and pull the weights up being sure to squeeze the shoulder blades together as you do it. Good Luck!
