I can agree that there may be ways of determining how rational a suicide is, but only hesitantly because to my knowledge, the science of what constitutes 'rational' is either non-existent or very poorly established - it's an imprecise term. Moreover as you point out there's a difference between rationality and morality, which is exactly why I think that rationality is irrelevant when thinking about the permissibility of suicide.
I agree that probably very few have killed themselves in a lost bet scenario as described, but should it be disallowed just because it's a rare scenario? I personally don't think so. I also think that small misfortunes like that can be triggers at the end of long line of stressors causing ctb, for some people. For example, David Katz committed murder-suicide after losing an NFL videogame match in Baltimore, 2018. Should a suicide be disallowed because others regard the given reason(s) to be trivial? I personally don't think so, because triviality is subjective.
The topic of the rationality of suicide is essentially a philosophical one since the subject is normative, not empirical. It's true there is no single, all-encompassing definition of the concept of rationality that is universally accepted yet most philosophers writing on the topic usually agree on some core principles.
There simply is no way to establish the value and meaning of life as this cannot be ascertained in an objective manner: it exists solely in the mind of the individual holding the particular opinion. One person's heaven is another's hell and vice versa. Which is why I will never pronounce judgement on others' CTB-decision: how can I know what's best for them? Or anyone else?
As to the connection between the rationality and morality of suicide: imo that connection is hardly practically relevant aslong as the anti-choice stance remains merely moral and not political/legal. One may dissaprove of another's actions in the moral sense but to that individual it'll make little actual difference provided he/she is left alone (behaviour that is generally deemed to be immoral is not necessarily illegal) while a legal prohibition (following a political decision) however subtly and deceivingly formulated will have a very real impact since it'll likely lead to meddling by the authorities and others and even incarceration of the suicidal.
Obviously such weighty topics would require a lot of time, effort and space to discuss properly but to me it would seem that in order to effect real chance in the legal status of suicide lobbying for the legalisation of rational suicide would be far more acceptable to the general public than the libertarian stance that all suicides should be allowed.
I'm sure there are people who in a fit of despair kill themselves while their situation didn't seem to warrant it and the urge would probably have passed given time (for the record I do not consider CTB in the case of long lasting mental problems to be irrational). I have been in that situation myself (wanting to die and even attempting once though objectively the situation wasn't hopeless) so this isn't a mere academic position. I consider myself to be a fairly rational individual but it would be folly to disregard the well-established fact humans can think and act quite irrationally and a wise man once said no man can be wise at all times.
It seems in that type of situation such individuals are usually glad to be rescued (not to mention their family and friends) so I'm not sure whether allowing such suicides would be a good idea. Even if I'm generally in favour of legalizing suicide. Which is why I would advocate for a waiting period and other measures (such as mandatory talks and being offered assistance in dealing with one's problems) to ensure only those who really want to die and who have thought it through would be granted their wish.
I've been on this forum for quite some time now and my general impression (a mere impression which I cannot really substantiate is that usually members who seriously contemplate suicide do not fall into the category of impulsive/irrational decision making. If one has suffered for years on end and nothing helped I honestly don't see why it would be irrational to want that suffering to end. This is not an endorsment to do anything (it's really not up to me to advise anything nor will I) but I cannot help feeling a great deal of sympathy for people's plight (apart from the occasional attention-seeker people who end up here have usually suffered immensely) and a certain admiration for those who rationally decided to call it quits. Admiration which is absent in the case of those who seem to foolishly throw their life away. This isn't based on a pseudo-objective evaluation of their circumstances (imagining how I'd feel in those circumstances is hardly relevant and I'm sure some people suffer greatly from causes that wouldn't even register for others) but on my impression of their state of mind in combination with the generally accepted criteria for rational suicide.
As to the hypothetical example of the lost bet: obviously by itself it would hardly if ever be enough as a motive for suicide which is why I wrote "I hardly think anyone has actually killed themselves
solely over losing a small bet" (italics added).
I think it would be intellectually dishonest to focus on the last nuisance and disregard all that suffering that preceded it.
Obviously murder and suicide are completely different things and I hold anyone in contempt who first murders another and then kills himself to escape justice. Wanting to escape misery that one didn't create is quite understandable but murderers are vile human-beings and while I wouldn't call their suicide immoral simply because they also committed suicide I do feel them evading justice is somewhat unsavoury. Interestingly enough David Hume took the opposite view: he claimed that for a criminal facing the death penalty it would be both beneficial to the individual and to society it they would cause their own demise.
As to your question: I don't think it's up to me to decide that. I've provided my view on the matter but I'm not narcissistic enough to claim I'm the ultimate authority and my opinion should become law.