By nature of our position as Renxiang's retainer and her needing clans completely loyal to her, Qi and GG are meant to build their own clans and are therefore unavailable to marry into anyone else's clan. Thus any suitor to Qi would have to marry into her clan, stay in snow blossom and any children from their union would be Ling's. Thus, having so many high ranking suitors to a mere baroness is practically unheard of.
Count scions tend not to marry into clans under viscounts, and it'd be shocking for a ducal scoin to marry under the count level so Xuan Shi's suit is basically completely unprecedented and would be considered a massive slight to Xuan Shi and a dishonor on the Xuan if it wasn't for Zhengui's existence making it possible. He even mentions when making his suite that Zhengui's existence makes it possible. Probably both because the Xuan want connections with the only other clan with Xuan Wu in the empire (and the possibility of getting some of Zhengui's bloodline to help get more Xuan Wu) and the fact that anyone who has a bound Xuan Wu companion has automatic status among the Xuan.
The counts are allowing it because Qi is an obvious high flier who is close to the heiress and the Xuan because of the aforementioned reasons, along with Xuan Shi being unimportant and isolated member of his clan who clearly wants this.