you raise many points and I will do my best to address some of them.
First: While I disagree with you, on what would be likely to happen in the situations that you describe, assuming that the people in question behave like real world people. You are perfectly within your right to declare that this is how things work in your game.
In general you tend to assume that magi will do a lot of things "because it is for everyones best" but this is a fundamental misunderstanding of how groups behave. Specifically the misunderstanding is that groups behave as if it was a singular entity. The order is a group in the sense that it makes sense for our minds to categorize its members together. The orders is not a group in the sense that its members all necessarily think as members of a group, such as you would expect say, with a beehive (not that bees think in the conventional sense of the word). Members of the order are individuals first and foremost and will act to further their individual interests, even if those interests conflict with the wider interests of the order.
Thus you assume that many breakthroughs will happen because "it would benefit the order", but many of those breakthrough would benefit the order at great cost to the individuals who made them. Will magi (as in: will large numbers of magi) be willing to sacrifice themselves to further the interests of other magi? Not in the setting as presented in the source material and not if they behave the way real people behave.
On the topic of integrating the combo of fertility magic/fertile longevity ritual. A number of things could happen. One of them is that the magus population explodes, and I think this would likely happen. However the effects of this are debatable. Would the magus population explode so fast that magical resources become over-exploited before a solution can be found? (assuming that a solution exists). Maybe, this is certainly the conclusion reached in this thread. However there is a crucial detail to remember: Malthus was wrong about his theory applying to humans. It is also conceivable that the magus population increases but slowly enough that magi have the time to foresee the problem and to seek out solutions, such as by looking for breakthroughs that allow for the protection, creation and strengthening or both auras, regiones and vis sources, or alternatively breakthroughs that reduce reliance on those. It is worth considering that in this scenario, most of the extra population will be blood descendants of established magi so the established magi may be more inclined to be sympathetic to the plight of these magi.
Personally I expect that the population of the order will double some 35-60 years after these combined breakthroughs, but I dont expect it to consistently double after that. Some magi will not want magical children, some will want one, some will want more. but it still takes time to bring a child in to the world, let it grow to 10 years and then train it for 15, before the cycle can repeat again. Personally I expect it would average out at about 1 magical child per magus.
After the first doubling I expect things to slow down since the ones having children will be the new generation of magi. Over time this would IMO lead to something like a 5-fold increase in the population of the order. This is, again, by my estimation a lot less than what is necessary to exhaust the resources of ME. It would however be enough to make the order a much more diverse and lively place since there are simply more magi around to come up with new ideas and to go on with new ideas proposed by others. These new magi will also on average be better than the ones that came before since their parents can choose what virtues they will have which will in turn cause many more children with high INT and COM as well as affinities, puissants etc.
But this takes us back to a key issue. If you are an old magus, looking at this breakthrough that will certainly make you irrelevant because you will not have the perfect combination of key virtues and meaningless flaws that are caused by the fertility ritual. Would you simply accept your fate and become a meaningless relic of a bygone age or would you struggle against it? on average people prefer the latter.
Otherwise I expect that it would be less dramatic than you suggest because all of these new options would compete with existing ones. Would you rather spend a bunch of seasons studying your specialty or would you traipse around learning new, increasingly more niche R/D/T's by going through initiation after initiation accruing ever more incapacitating flaws on the way and/or indebting yourself to skilled mystagogues?