Not that easy, Infected Ground requires jumping and there the whole locked room and other funny stiff, can you get the Optional in Infected Ground solo? I do not think so.
The entire missions would have to be redesigned for solo players in terms of objectives and difficulty, you cannot defend the Kang and blow up the Cube Shipyards at the same time.
Yes, some reworking would need to be done. But the new STFs introduced the bridges on normal which would work for BOFFs and who says there needs to be an optional for the solo version?
Stuff like teh kang defense I would cut back out and return it to the original scripting where you had a timer to destroy as many shipyards as you can to cut down teh number of enemies before the fight.