So, just to clarify, when you say marking, you mean her anal glands, correct? So, she is an indoor/outdoor cat. Is your other cat indoor only?
She may just be territorial, and animals will express their anal glands when they feel stressed, anxious, or fearful. Its just a natural response for them.
Anal glands can be surgically removed, however, it is not a routine surgery and there can be complications.
I would suggest having her in to your veterinarian's office to have them manually expressed every 2 weeks to cut down on the times she is able to express them at home.
If she was adopted from a shelter and she was a stray, that may explain her eating habits - she may be used to never knowing where her next meal was coming from. When she's outside she doesn't understand that the other food bowls aren't hers and she may just be trying to get a meal when she can.
I hope that helps - let me know if you have other questions and we can go from there!