Returning to the topic of shoes for dicks:
Cobbler (shoemaker) and milliner (hatmaker) are grouped together here because they’re both things that go on the “head”, adding onto Harry’s preoccupation with the head / “head”. The rest of this paragraph relates to sex too.
Dabble means to ‘make wet by sprinkling a liquid’, so him dabbling in 'accessorization’ (things applied to the dick) is a joke about him using a condom. A 'bit’ is also a tool applied to the head of a drill to bore holes, 'whizz’ suggests fast movement and the needle and thread relating to sewing / sowing, once again evoking condom usage.
'Amateur productions’ is teen pregnancy, and their unappreciation of Harry’s accessories is a distaste for condoms. It seems Harry wants to prevent unwanted birth due to John abandoning him.
And as Imode says here, Harry isn’t scared of being in public so much as he is speaking in public, and speaking is birthing the baby. It’s shown in this panel too, as he nervously stares at the microphone held up to him. Harry’s reluctance to perform in productions despite wanting to is because he’s scared of accidentally creating a baby that he doesn’t want. Avril putting child-star together also makes me think the stars he’s associated with are supposed to be children. Makes sense to me, since the most notable stars in homestuck are suns, a homophone of son.
And of course within the star (sun) embedded closet is a figurative son embedded with a little star of his own, who has also been neglected. Harry then, ironically, decides to become involved with his bunny again at the same time John decides to become involved in his son’s life. Repeating cycles…