Can't unscrew metal p-trap nut
Yes, once removed you may use a plastic trap assembly to replace it.

You will need a bigger tool, like a large pipe wrench or large "Channellock" type plier. It won't be easy (upside down headfirst in a cabinet with restricted space) but that's why plumbers are well paid. Try soaking the nut thoroughly with a penetrating oil overnight.

They make offset wrenches for this type of job, but they are expensive. I have never needed one, just a big pipe wrench.