It was explained pretty explicitly by Evelyn. Anyone she touches on the present day, she could scry into the next following day. The host is selected at random, but she's learn to select her host at will from those who she's physically touched.
Example: She makes physical contact with 5 people on the 18th... When she goes to sleep that night, she'll scry into the entire day's events of the 19th with full sensory perspective of her host. ONE of the five people she touched will be selected at random unless she chooses one of the five at will.
The scene where MC scrys into Marina was simply to illustrate how Evelyn's power looks from her perspective. He might have touched a number of people that day and he was trying to scry into Evelyn per her advice but he failed. So his host was chosen at random and it happened to be Marina.
He did touch her before acquiring the power, true. But based on the rules we know of her power, it was within how it's expected to work. He did touch Marina that specific day and he scryed into her next day while he slept.
That the time it is decided is "a day" is what Evelyn
THINKS applies.
That it did not happen for days prior (and not all people look upon a calendar daily i.e. Evelyn did not necessarily know whether it was sunday or saturday when she scried that barista ...) does not mean it
can't happen.
For all we know she could be scrying a dude for today she accidentally brushed against in the bus last week and her power only
NOW decided to scry him.
Every power is different. And users knowing exactly how their own power works at all times are rare at best.
Amber for example
knows that she is permanently invisible. Truth is it apparently can be controlled as she went visible briefly at kissing MC.
And btw. there was no mention of the current day MC scried. It could very well be that this was the second day at school for her (first day she got her clothes nicked and the day would stand out) the very same as if it was the day that is about to happen. It could be that Evelyn uses the "standard version" of the power that only works on this day and MC is "special" (apart from having a telephone pole as a dick) and accesses the "improved version" instead.
And no from what is known of that power it is not said that "it was within how it's expected to work".
Your assumption is based on Evelyn, but remember she had it way prior.
I see it as the mutagen is transferred via touch, much like todds imprint, and that action activates the power. Thing is at the point MC transferred the mutagen it could not do that.
It is like trying to eat a boiled egg for breakfast before you boiled the water.