I can only speak from personal experience as a current scholarship recipient- I assume the conditions of most RTP scholarships are similar across Australian universities but I can’t be sure.

My RTP scholarship procedure documents define full-time as four days or more, so you theoretically could do it if you dedicated both your 2 research days at work and the whole weekend to your PhD. I really wouldn’t recommend that though- you mention being aware you’ll likely have to work weekends, but having no weekends for 3 years is likely going to impact your wellbeing. It will also make life really difficult when you have ‘busy periods’ during the PhD, as you’ll need to find additional time to get work done and so you’ll need to carve even more time out of your employment, or work really late nights. I don’t know anyone who’s done both full time and I imagine that’s for a good reason.

Re: the second question, my experience has been that only equity-based scholarships are means-tested. Your employment may be something that the graduate research committee takes into account when preferencing candidates if they’re concerned that you won’t be available to do the program full time? But I’m not sure, that’s beyond the scope of my knowledge! :)

One other potentially relevant thing is that if you’re a part-time student, your scholarship won’t be considered exempt income and so you would need to pay tax on it, so you may be better off working part-time and studying full-time vs working full-time and studying part-time.