My experience indicates that Ink is probably not going to let you in for free unless they have to, like when their index gets really stale.
Think of getting into free Ink as a random chance. Put up a site a wait. It may get in soon, it may get in after several months, or it may never get in.
Getting spidered is a first step to getting in for free but is not a guarantee at all (I've read where having good incoming links helps get you spidered but I can't confirm that).
Also, my experience indicates you're just as likely to get a paid site booted (unpaid pages removed from the index and paid pages sent to the bottom of the SERPs) as you are to get an unpaid site into their SERPs.
I don't see any method to their madness, and that may be by design ... keep you guessing so you don't figure out how to manipulate their results. It could also be that their algorithms and systems are not robust.
The guessing is now extended to what they will do when, and if, Yahoo drops Ink under their hood.
My experience does not cover the XML feed, it only covers free and pay-per-page indexing.