I see two viable paths here. The key is that the Velaryons control too many dragons. Without Daemon we must have them as allies. That means either we marry Laenor, or our father marries Laena. Viserys marrying Laena option is very similar to the Plan Not Great Not Terrible, but with more optimized selections.
[X] Plan: Second Mother to Our Siblings
-[X] Matters of Law and Coin
-[X] Try to convince him to support a marriage alliance with the Velaryons, but delayed. We need their dragons as a hedge against Daemon. Considering Laena's age, a betrothal while delaying the actual marriage for four years is reasonable. It will give your father time to grieve, and it will ensure that you will be 19 years older than any siblings, allowing you to marry your children to them if Laena has children, avoiding any future dynastic conflicts.
-[X] Agree to support their efforts in exchange for favors (Select as many as you want but they will likely refuse if too much is demanded)
--[X] They agree to delay any marriage for four years. This is far safer for Laena than trying to have children too young. It also allows Viserys time to grieve, making him more likely to agree.
--[X] They agree to support your inheritance to the throne, and you agree to wed your heir to a child of either Laena or Laenor's. (If you have no children, of course you will name a sibling heir).
--[X] Lord Corlys agrees to support your efforts to attain a seat on the Small Council
[X] Plan: Political Marriage is our Duty
-[X] Diplomacy and Etiquette
-[X] Try to convince him to help delay your father's decision on who he should marry.
-[X] Try and convince them to drop offer with Laena and have Laenor marry you instead (Will increase Stress if they accept)
--[X] Lord Corlys agrees to support your efforts to attain a seat on the Small Council
The first plan leverages our primary emotional connection we made with Otto. It's too soon, let a man grieve. While recognizing that we need the Velaryon dragons. While at the same time presenting a plan to avoid future dynastic disputes. As 19 years older we will functionally be a full generation ahead of any siblings, and can arrange matches as needed. Taking such an approach means we have much less need for diplomacy, and we can instead look at things like Stewardship, improving our chances for a Small Council seat.
I like it much better than the current version, which does not delay the marriage, and as such I think greatly endangers our inheritance. Being 19 years ahead of any children will make a huge difference, and already suggesting a marriage alliance of our children means that the Velaryons will be fully on board as our supporters. I think the current plan is too naive about what will happen if Laena marries young and produces a son. In that case it's the Velaryons that will become our enemy, Blues vs Blacks, and unless we marry Daemon (or Laenor) it means we will be the one with the disadvantage in dragons. Far worse than the one that the Greens faced in canon.
The second is a much more straightforward action. We gain our fathers support by accepting the duty instead of him. It means we will still need to take action in the future about our father's marriage, but it secures the Valeryon dragons behind our inheritance. Yes we end up with stress, but that's the cost of being an heir.
I suppose there is also the third option:
[X] Plan: Political Marriage is Both our Duties
-[X] Diplomacy and Etiquette
-[X] Try to convince him to help convince your father to choose another bride
--[X] If your father is going to marry for the good of the realm, then it should bring the realm something: Propose Deria Martell as a match.
-[X] Try and convince them to drop offer with Laena and have Laenor marry you instead (Will increase Stress if they accept)
--[X] Lord Corlys agrees to support your efforts to attain a seat on the Small Council
This would be going for the full option to make sure there are no threats to our inheritance. Dance is cancelled, and we can focus on the rest of our interests. While accepting that our father must remarry sooner rather than later, at least Deria Martell will require a long time to negotiate, giving father at least more time than he has now. (Plus it also means no threat to our inheritance, and opens the door to securing the Stepstones permanently, and perhaps eventually folding Dorne into the realm).
EDIT: I guess this plan is close to the third one of mine, so I'll approval vote it for now:
EDIT2: Since my plan caught up in votes, no longer any reason to approval vote.
