This is an interesting thread, but there are some commentaries to be made.
First of all, the OP refers to a problem-riddled board. A weak France, an idea-less Italy and a weakened nation that creates a strategic imperative.
About that last thing; destroying a weakened nation is not necessarily immoral. In fact, it can be necessary to restore the balance of power on the board. After all, if there are, for example, six centers to be divided by either 2 or 3 players, it's the best that it be done with 3, so each player has 2 centers and some balance is maintained. This balance is lost when 2 players divide the 3 centers among them. Also, the result is the same.
When you're in a situation like you describe, feel free to come to the aid of the weakened nation and then, when you find out you can't help him (or you can, but you pretend not to be aware of that), simply ask for permission to take some of his centers. Honestly discuss the situation with him; that he can't be saved (tell which moves you want to make and how they can't salvage him), but that he can still influence the game by letting the one who was most honest to him have his centers. Also, you can promise this person a survive (if you really managed to establish a relationship of confidence.) Just tell him you will protect his capital city until the end of the game and that you're aiming for the solo. It wouldn't be the first time for me that a deal like that actually passes. I follow through on this, too. I reward loyal nations.
Then about lying; my playing style is very defensive in this respect. In my profile, you'll see just one win, but also just one defeat. Try to maneuver yourself in a position where you can't be lied to. This is very difficult and requires great skill, but it's possible.
Never open up completely to an ally and certainly don't open up more to him than he does to you. Always make sure you have good agreements on your frontiers, and demonstrate willingness to respond to what you perceive as aggression.
Don't hesitate to lie to players who lied to you first. Then tell them that you believe you had your revenge, that you're both equal now.
Then, you lie to him again. The second time, tell him that you had no choice but to lie, because although the first lie has been avenged, you were in a bad position all the same, because of the bad initiative the other person took not to keep working with you.
Then, you lie to him again. The third time, you explain that you simply don't accept being lied to and are going to kill him. Be sure to laugh, slap him on the shoulder, call him a cool guy and everything. Apologize, but with a smile on your face. You are now the alpha and you have to play your role mercilessly. Make sure this person does not make the same mistake in a future game.
Of course, you should only have to lie once and then you're going to kill the guy.
Learn how to play the victim. Because when you're being lied to, you are in fact the victim. Point out to other players that you've been stabbed, and show the moves which prove that you've been stabbed. Force the liar in the defensive.
You'll be fine.