Welcomes !!! glad you are ready for a journy
1) It is not needed but a well made banner lets your work stand out more and be more appealing to subscribers so its reccommended you have one.
2) More or less, yes. Mostly to remind people of your patreon or that you take tips (tips are ink on tapas but you can only recieve ink after you reach a certain number of subs)
3)Mostly due to it being more a comic site first hence the newer better artwork of your later episodes are seen as more a draw to audiences. But I dont think it matters too much thou. I just leave it as default cus it doesn't affect me much.
4) I'll say no harm is having one or not having one. If you want people to more easily follow you else where might as well have it if you want.
Also a thing whats a graphic webnovel? if you mean just web novel with some pictures, well yeah I believe you can have up to 5 pics per episode in a webnovel lol. I tend to have at least on illustration per episode for funzies. If you mean web comic.. I recommend making a web comic directly if that's your goal. cus tapas rarely adapts its non imported western novels to webcomics
. Even the biggest creator on the site Kiss it better had to commission other people for make their short halloween comic.
Overall the community is willing to give advise and help anyway they can, so dont be a stranger when it comes to asking for advise and help