According to http://www.usccb.org/issues-and-action/marriage-and-family/natural-family-planning/resources/upload/Reproductive-Technology-Evaluation-Treatment-of-Infertility-Guidelines-for-Catholic-Couples.pdf it is not in accordance with Catholic teaching except under circumstances you mentioned. No mention in the article about relative gravity of the sins of IUI vs IVF which I would expect to be quite disparate. If you find something ping me.
Good find thanks.
To me, the document you found seems very helpful with regards to the issue of IUI. It says that procedure is neither “approved” nor “disapproved”, so, as far as I understand Canon Law (disclaimer: I’m no Canon lawyer), it would be acceptable for a Catholic in good conscience to elect such a procedure, after close consultation with a trusted priest and much prayer.
Pinging someone who may have some educated insight to add.