The amount of misinformation and lack of technical knowledge is staggering.
Firstly, no good employer will just at all if an employee doesn't have a suitable laptop for this interview.
I have interviewed plenty oh people throughout the pandemic. The only thing I'd expect is the candidate to be honest with me & we would do everything we could to find a solution.
Secondly, I agree with asking for help from your work coach.
In relation to joining from your phone, the interview itself from your phone would be fine. The presenting, less so. It will be too small on screen & hard to really deliver a quality presentation that way - technically it's possible; in practice you are likely to be insufficiently confident & that will affect your performance.
You do need a suitable device, but I would be confident that a good employer would help you. I know I would.
In terms of purchasing & returning, don't do this - even if it's allowed, you would need to purchase the software too, and once that's installed, as it needs to be, returning will be harder.
Going forward, it would be great to put in place a plan so that you can in time buy a laptop. I know that's not possible at the moment.
Best of luck with the interview 💐