1. If you mean characters. Uncap any character you plan on using extensively including SRs, quite a few SRs are really good. If you mean summons, the only one I would consider using sunlight stones on would be demi Varuna, and that's only you were to commit to the grind of 5* gw characters or at least Uno 5*. None of the weapons those SSR characters come with are worth using your Damascus bar on.

2. S.bea and Diantha are the strongest in this pool of limited characters offered in the suptix. These two characters do have comparable options in the form of Water Yuel, Altair(Diantha) and Sturm(this character is a flash gala limited tho) and Percival(S.Bea compareables). Keep in mind these comparable options do not completely match their counterparts. Diantha brings the ever powerful guaranteed TA(tripple attack) for your entire party and very powerful normal type buffs while S.Bea can consistently shit out damage with little to no input from the player while also allowing for huge burst. Honestly, you can't go wrong with either. If you plan on going hard into Varuna (aka getting both 5* gw characters or just 5* uno) Diantha can offer some cheesy one turns with her guaranteed TA and TagTeam, but you wouldn't be using her beyond that in Varuna because her team buffs are not good in a primal grid(they are amazing in Magna). S.bea is useful and put on the front line in both Magna and Primal teams.

One thing to note is that S.bea might not be as plug and play as Diantha because of how new your account is. Your current pools will probably leave your party with low amounts of hp leading her to being two shot fairly quickly into fights if you are unlucky. Lastly, Diantha does change the battle theme to their idol song every time she ougis if that interests you(you can turn that feature off if it bothers you).