I have a feeling even though eBay owns Half and they share some integrated systems, it's still pretty much two separate businesses.
I did notice that when I went to leave feedback for an item on one of my new accounts, I had to "activate" the account for eBay where they had a computer call me and give me a PIN to punch in to activate my account before I could try to leave feedback. Then it wouldn't let me leave feedback because your eBay account has to be active for more than 5 days before you can leave feedback.
So I assume if you just don't even bother to activate the account on eBay or Half to leave feedback, eBay won't really think much of anything about the account.
And besides, if mega re-sellers like Alibris can have 15 different accounts, why can't you? Until you do something fraudulent or against the rules (like using one of your accounts to leave positive feedback on another account owned by you), what does eBay care? Aside from the free shipping loophole, that is.
And if the people running Half.Com really cared about cutting down on the use of this code by new accounts, they could have done so by now. Surely someone has looked at the numbers and seen a drastic rise in new accounts with identical shipping and residence information created via Half rather than eBay, whose only action has been one order with free shipping.
I think the credit card denial after several orders across several accounts is probably a fraud protection thing rather than a "hey you used this coupon too much" thing. Some guy on Slickdeals said after many orders it started rejecting his phone number, but he just plugged in a Google Voice number and continued on.
Besides, if you're worried, just wait until all of your items are in and everything is fine, then delete your account via eBay, which will delete it across both sites since they share the same database.
http://pages.ebay.com/help/account/closing-account.html I probably wouldn't recommend closing them all at once since there is a waiting period before the account is closed and that might raise some flags.