the MH750 has 3 rings on the jack as a opposed to the 2 rings on the connector for your other headphones am I right? Having the extra ring on it (meant for the remote) may be what is adding the extra tightness when inserted. I'm not sure how to solve your AUX problem but you may need to try headphones which have a built in remote like the MH750 so that it has the extra ring on the connector. This may also be a defect, as I can not confirm since my device is still on it's way.
EDIT : After receiving the phone this happened to me once. You have to play around with the smart connect settings o disable it all together. It doesn't seem to be hardware related.