I'm pretty sure the staff is working on addressing this issue, so that:
When a locked episode gets posted, non-supporters don't get notified.
When an episode gets unlocked, non-supporters get notified properly in their reading list.
Because otherwise, it IS really annoying.
I've worked around this issue by unlocking an episode precisely when I post a locked episode, so every reader, supporter or not, has a new episode to read when they get the notification. Supporters would see the locked episode; non-supporters would get to see the one that just got unlocked. It's not a perfect solution, though, and I can't wait till they fix this.
It's interesting that we seem to be total opposites in almost every issue you've addressed:
Hiding the extra material from the majority of your readers makes no sense to me, especially if it enriches one's understanding of your story/characters! I'm all for reserving the high-res version of stuff for supporters, whether it's extra material or the comic itself (like cover images or text-free versions of those extra cool-looking panels). Those are neat to have. But if you're not a supporter, not having access to that stuff does not affect your reading experience in any way.
This actually annoys me as a reader (just a little). Why? Because following a comic on their own site is a pain. I'd rather have locked episodes here on Tapastic so I can at least get notified.
Personally, I don't see any of the perks as an incentive to support. It's more of the creator's token of gratitude. The quality of the comic should be the biggest incentive by FAR: it's so good, you want to support the creator even if you don't care for any of the perks.
I'm all for spending money on quality stories; it's why I buy physical books, and I would pay my favorite webcomic creators for the same reason (as soon as I'm able). But little perks like request sketches or whatever? If that's what I'm after, I'll rather commission an artist to draw me a sketch. Now, I offer request sketches and such to my supporters not as carrots, but as a token of gratitude. No matter what the perk is, it's insulting when you look at it as the creator dangling a carrot in front of your wallet.