I'm not arguing that Ranma is honorable only that he has some type of honor code. While telling his mother is the most honorable thing it. He still has to have some honor else there is no threat from sepukku, since he is supposed to make the first wound by stabbing himself in the stomache and it is not a threat to someone without honor since he could just run away or not go through it.
The fiancee's are his father's fault and he only has two officially and Ukyo's is on shaky ground seeing how Uyko's father already knew Ranma was engaged and Genma gave Ranma the choice as per modern omai and Ranma did not choose Ukyo he choose Okanimayaki.
Not really his choice to string them along there all all kinds of possible repercussions in denying them as well and he did try and get rid of each of them on at least one occasion except for Akane.
If he had had the guts to tell Ukyo and Shampoo, mabye it wouldnt have gone so bad.
Actually he did not want to marry Akane at that time he attempted to get more time in volume 37 (VIZ 35) and he tried to start a fight with Akane to delay the wedding and get more time in volume 38 (VIZ 36). It could have easily gone much worse had he told Ukyo and Shampoo (we do not know how they would react or to what extent they would go also even if the marriage had gone through that would not necessarily stopped them from perusing him since Kodachi and Shampoo at least wouldn't let a minor thing like him being married deter them) and he wasn't given a chance at that point seeing he didn't know about the wedding until moments before it.
Has it every driven Ranma to do anything, he wasnt going to do?
Unknown the reason for why he is doing the stuff is not always given it could very well be the reason he does it in the first place.
Hes got a roof over his head, and three meals a day. A real life. He is quite happy living with the tendo's. Honour does not keep em there.
None of which matters to him and could be easily gotten elsewhere. What Nermia does not have is a cure to his curse which is not in Nermia it could be argued that he stays in Nermia to honor the engagement or because it's as good a place as any other we simply are not told his reasons.
My point is we can not say with any certainty as to whether Ranma is honorable or not because we do not know what, if any, honor code he subscribes to though there is some evidence of him having an honor code of some type (if nothing else some type of martial arts code that entails helping others and fighting monsters).
Your argument Ranma is not honorable, mine is from evidence in manga his honor is unknown and can not be conclusively determined since we lack evidence for or against though there is some evidence that he follows a code of some type but the full extant of that code is not told to us. Of course a lot of this is speculation as well as personal interpretation of events (some events can be interpreted differently like the bet with Ryoga seeing how it can be viewed as falling in first or being defeated) as well as biases (both character and cultural -not saying they are negative or positive biases but there still are biases, I admit to being some what biased in Ranma's favor seeing how he is my favorite character in fiction). My personal opinion is Ranma is more moral than honorable though he does subscribe to some honor code what it entails and to the extent he follows it is unknown.
As for interpretation (not really my opinion but is a possible interpretation) it could be argued that by not marrying anyone and stringing them along is the honorable thing. By not making a choice he is maintaining the situation until he has a way of dealing with it (or becomes a legal adult and has more options). Some could even argue that the only honorable solution is to the fiancee mess is to commit seppkku and appease honor, where as others could argue that the only solution is to marry no one ever, and others could argue the solution is to go ronin foresake the family name entirely, and others could argue that the honorable thing to do would be to have the Tendo's adopt him into their clan making all claims to him in the Soatome clan void (this is unlikely and not a very good solution since it is viewed as a feminine practice and would effectively kill off the Soatome clan).