So does my Handy Haversack Belt Pouch count as a constant item?
Yeah, I think it would... unless I said otherwise somewhere...
You could always enchant it further, though. So it's not just a handy haversack belt pouch. Wishes can also be used to increase the value of existing items, up to the 15k limit.
As long as it's a wish from the spell or through a magic item. Creatures with Wish-SLAs, like an Efreet, can't grant an enhancement to an existing item. Also, apparently you can't wish for an item with wish in it, according to those rules. The whole chain that would result in these wishes is fairly confusing to me, actually. But if you say it works, then I'll believe it.
Meh, what's the difference in 1) upgrading to a total value of 15k, and 2) wishing for an entirely new item that has the exact same properties as the old one, plus the new additional properties (total value less than 15k)?
Flavor. I like the flavor of being able to upgrade better, so we're going with that.
And the way they expect you to get "infinite" wishes is to call an efreet, and use the last of the three wishes he grants to call a second efreet for more wishes. That's technically not wishing for more wishes. You're just emulating Planar Binding.
You guys could do that if you want, but I like the "flavor" of having a powerful benefactor just provide you with items/wishes/etc.