In short: yes, this is risky!
1) since i do not know where I will depart to until closer to the date (I will already be in the US) I plan on buying a one way ticket, will that raise red flags?
Timatic, the database used by airlines, states the following about VWP passengers:
Passengers must hold a return/onward ticket.
And when checking in for US flights in particular, airline staff is bloody strict, as they face huge fines and possibly loss of licence to fly to the US if they don't get it right.
So in fact, you have to buy a refundable return/onward ticket to anywhere but Canada, Mexico, Bermuda or the Caribbean, or you're not getting on the plane to the US. That isn't to say, though, that US immigration will ask for a return flight confirmation themselves - never happened to me.
2) Will I have any issues in returning to the US after a couple of weeks i spend in either AUS or the UK?
Most likely yes. As well as your return/onward flight confirmation (real or "ploy"), attach, in an A4 soft binder, every single piece of evidence that you have of your ties to Australia, including a good signed and stamped letter from your university with their contact details clearly stating that you're in good standing and expected to be back for a graduate job at a certain date. Be prepared to present the documentation if asked, both on your first and in particular second trip.
The following is what I got from the CBP via E-Mail.
Meaning: your trip is doable, but you should be prepared.
Other answers have suggested applying for a B2 visa, which would eliminate the need to buy a refundable return/onward ticket, but costs much more, as much as outlined by @phoog. You are also subject to much closer scrutiny this way, and if they deem your ties to Australia to be insufficient, you'll be refused with no refund and it is unlikely you'll ever be able to enter visa-free again.
As such, getting a visa only makes sense if you would like to stay for more than three months at once (as with a visa, you normally get to stay for six months on each entry). The CBP did not mention this being the preferred path, but simply that proof of ties to Australia should be prepared for presentation at the border