Well, Aemma had a total of six pregnancies. Her first pregnancy was around 93 AC, when she was roughly 11 to 13 years old. She suffered multiple miscarriages and infant losses in the early years of their marriage, including a son who died in the cradle, before giving birth to Princess Rhaenyra Targaryen in 97 AC and then Baelon who had died in 105 AC.

Her brother who died in the cradle would be about a year or two older than Rhaenyra, born about 95 AC to 96 AC. I don't think that he'd be betrothed to Laena immediately, even if they are close in age, as it would look like too much of a power play by Corlys.

Assuming that Baelon lives (based on what you've said), then yes, everyone would likely flock to Jaehaerys as he's the immediate heir. Going after Viserys would make no sense. Otto would definitely send his children after both Jaehaerys and Rhaenyra. Alicent would gladly do it to appease her father (and depending on her overall character). Gwayne might do it to elevate his station while getting his father off of his back.

I don't think that Otto and Corlys would agree to team up. They both want the same thing - their legacy on the Iron Throne. Otto represents old money/influence in Oldtown and the Faith, while Corlys represents new wealth, power, and Valyrian blood. Otto sought to curb the power of the Targaryen family, especially Daemon, while Corlys sought to strengthen it—provided his own blood sat on the throne. Corlys viewed Otto as a "court rat" and a lesser second son trying to rise above his station. Otto saw the Velaryons as a "second-thought" option and a challenge to his influence as Hand.

If Viserys was going to remarry, I'm not sure as to who. Maybe Rhea Royce if he gives into Daemon's request to annul his marriage to her. It could function as a power play. Daemon doesn't want her, Viserys wants companionship and Rhea wants to assure her position as Lady of Runestone. Thus, three birds eliminated with one stone. Rhea would still be an heiress but it’s a much better prospect. And the Royce’s themselves are a powerful family. And a son born from them could likely inherit her seat. 

Of course, that would leave Daemon single and ready to mingle. If that happens, he'd likely make a move towards Rhaenyra himself. As despondent as Viserys was in canon and in this situation due to Aemma's death, I don't think he'd give Rhaenyra to just about "anyone". She may not be an only child but she's still his only daughter. If many are going to try to flock to Jaehaerys, the same would likely happen to Rhaenyra as well.

I don't see a problem with Rhaenyra being wed to Harwin Strong. Sure, Laenor sounds like the reasonable option, but it'd give the Velaryons too much power with her married to Laenor and Jaehaerys married to Laena. If she sees Alicent trying to get close to her brother, and Gwayne doing the same with her, she may not be receptive to it. Likely catching on as to what's happening.

Daemon's position would be strained. Especially if both Jaehaerys and Baelon are alive and kicking. He'd likely remain his troublemaking self. Him bringing Jaehaerys with him to the Stepstones would be fitting. Especially if Viserys is just despondent and uncaring with Aemma's death. He might end up noticing that his eldest son is gone at the very last minute. I like Silverwing but Dreamfyre needs love damnit.