I keep thinking that the likely outcome of all this is that OP does one or two of her jobs in a slightly shit way for a while until they manage her out. She's mentioned leaving "when the heat gets too high" which sounds cool and counter cultural, hey, doesn't it?
What she is really doing is wasting loads of management time, probably having reviews, written warnings, blah blah. Wasting their time as she spins out the time they take to manage her out, taking advantage of legislation and practice - legislation which is bloody hard won, through bravery and strikes and pain and actual starvation of workers in the past. All the laws to protect workers from being taken advantage of, or summarily sacked.
I don't buy the thing about being super efficient in all jobs, if you're a more junior marketing person you will have things to turn up to and deliverables to achieve, and it will become obvious if they are done quickly or at night without the right level of input from colleagues etc.
The example above. If you're in an all day meeting at one job, and the other job wants you, presumably you just give an excuse or just don't show. Now, someone on my team could "not show" for quite a few meetings before I got pissed off enough to manage them more assertively. But I would start to notice, start to think about how to handle it, research the patterns, talk to colleagues. All additional to MY day job.
So I'd have someone not performing that well, or being flaky in team relationships and things I want them to attend. OK, they might be making money or completing their tasks, but all my team members also have cultural objectives and development ones and it sounds like OP doesn't GAF about that shit.
So I would turn attention to this low performing or patchy person and manage them more closely. I would start to ask for day plans of their week etc. I would follow up. I'm a good manager and I would ask myself if they were OK, if they needed support, if there was anything going on at home. A pain for me, more time I have to spend. Emotional labour.
OP would game that system relentlessly, getting more value for herself out of our work and taking more of my peace of mind and destroying a team culture into which she'd been welcomed in good faith.
So you can say yeah, capitalism, every person for themselves, dog eat dog... I'm afraid I see it as childish posturing and a distasteful breaking of the social contract.
Our employment rights are precious and hard won. If you think the 9-5 isn't an efficient way to work, and you can be more productive, then go set up your own consultancy and show us all a better way of working. Don't just steal from the collective till. I bet you'd be the first to bleat if your protections or sick leave or maternity leave vanished because of people taking the piss.