At the point of critical decision how many armies dod you get if you cashed your cards? It really depends on this, since you seem to be quite weak.
In any case, it seems that your best option is to sit tight. It is good to be worried about the future but only so far that your own presence is not under question.
You see, if you are going to become very weak, there is absolutely no point for you to become a hero and save the world. You are doomed by definition.
Instead what you do is to hang around. The other guys may suddenly start to invade each other. As long as you don't look like the next weakest player to conquer, they may leave you alone. At this stage of the game, preserved armies are far more critical than the continent bonuses. So don't worry about Africa, just get out of their way.
So if I was in your position, and I was getting a fair bit of armies, I will place them at the bottom of Africa and do nothing. Just making sure that I get my cards. I will stay powerful and wont give them any incentives by becoming valuable.
My moves after that depends on the attacks of the other players. I may decide to attack one of the weakest players and get his cards and go from there.
(This Risk Map Editor seems to be fun to use ..., I should give it a try...)