Recall the deaths in UK caused by Russian nerve poison...just a smear and it's all over.
You can bet your lower torso that USSS scans everybody while waiting in line for Rad Isotopes.
Absolutely. There are unconventional weapons out there that would accomplish it. She could pass through security (metal detector) unnoticed.
And she had her Q shirt made - made by whom, and for what purpose? Obviously, she wouldn't want the poison on herself, but she'd want to get it on the President (this is assuming that she sought to assassinate him, which is far from proven). A method of doing so might be a shirt designed specifically to shield her (or she could be wearing something under it) and also designed to release the agent upon pressure against the front of the shirt.
In essence, her shirt could serve as a container/delivery tool.
I know, I've watched way too many spy movies.