The crash happened. Only Riki and Rin were gonna make it out. Kyousuke, knowing this, regretted leaving them behind, knowing they wouldn’t be strong enough, and wished he could have made them stronger. Cue orb of light. Then he realizes there are 7 other orbs of light. Cue creation of the world. These orbs could be called regrets, they could be called wished, they basically mean the same thing because having some regret means they have the wish to change it. It could be said that the wish part was what created it and the regret is what fueled it.
I guess I assumed Kengo and Masato’s regrets were similar to Kyousuke’s, but what you said makes sense. Either way, all three of them basically come down to not wanting the Little Busters to be split up and for Riki and Rin to not be alone.
The 5 other girls have nothing to do with Riki and Rin becoming stronger. At least not at the creation of the world. It is made glaringly clear that Riki, let alone Rin, doesnt even know most of them all that well at the beginning except for as classmates. So then why would their dying wish be that Riki and Rin be stronger? And several of the girls run away from Riki in some way during their routes (Mio, Kud, Haruka). Why would they do that if their main goal was to make him stronger?
Riki and Rin do both have regrets, but they have nothing to do with the world itself until the end, when while running away from the crash, Rin creates a 9th orb of light, cue the Last Dream scene. During which Rin fully realizes her regret, but does not have it completely fulfilled: only enough to allow her to be strong enough to survive with the support of Riki. Cue hospital scene. But if you choose “It’s not enough” this is where Riki’s wish/regret finally kicks in. The world restarts again, Rin’s regret is fulfilled completely, and Riki, finally, fulfills his regret.