Yes so Harlequin does not feel JEALOUS but rather ENVIOUS, please remember that since it's very easy to confuse (even for me) and that would misinterpret Harlequin's whole character🙏
If it was JEALOUSY, it will mean that Harlequin loves Colombina and hates Pierrot because of that, but it's not the case*: It's ENVY as mentionned in the story: "...Who saw the light in the lover's eyes and felt envy gnawing [at] him from within." Harlequin felt envious of Pierrot since he didn't have what Pierrot did (i.e. Colombina's love and attention). He just couldn't sit with the reality that someone was happier and better than him so he wanted to take what Pierrot had. *He didn't care about Pierrot (for killing his love) nor Colombina (for killing her), he just wanted to have what he couldn't, and that made him happy to see that Pierrot is as miserable as him now. Harlequin is a pretty complex character for being Arlecchino. I wonder why is he so envious... Like leave my baby alone😭
Oh and about why Pierrot is not allowed to speak, it's because he's Pierrot! His attire might have been modernised but his role remains unchanged. Pierrot is a mime and mimes are famously known to imitate without the use of speech. So Pierrot has to remain silent to fulfill his role; it's as simple as that! But I do wonder if perhaps there is something else below the surface on why he cannot... Or maybe we're overthinking this!