1) I'm wanting to listen to HF amateur bands, HF Oceanic traffic and VHF Air Band. I'm happy that I'll have to use two different antennas, but what receiver would be best. If there is only a single antenna port how would I switch antennas?
It all depends on what you mean by Best. The RSP1A and RSP2 both have their pros and cons. The RSP1A has a 14 bit A/D vs. 12 for the RSP2 (which gives better performance in some cases). It also has some filter advantages the main one being the MW filter works for LF reception which your post did not state as a band of interest. However the RSP1A only has one antenna port vs. 3 for the RSP2. It is nice to be able to software switch the antenna ports. The RSP2 also has external clock output/input which is not that important for most users. I own a RSP2pro which I bought before the RSP1A was released. If I was buying today I would get an RSP1A and save some money. I suspect there will be an RSP2A before long that has all the features. It is easy to switch antennas with an external box. You can make your own or you can buy one ready made form any good ham radio supplier. Daiwa makes nice switches and for the budget conscious MFJ.
2) As well as monitoring air band I would like to track aircraft using ADS-B. Is this possible and could I track and listen to aircraft at the same time?
This is possible and SDRplay has an app note on how to do it and you can download the software from their site.
https://www.sdrplay.com/docs/SDRplay_AD ... _Guide.pdf. However it might not work on a Mac. I am not a Mac user so I can't comment. Mac can run Windows software so if you have Parallels or VMware you should be able to get it to work. Go to the SDRPlay group on Facebook and ask there about doing ADS-B on a Mac.
3) I use a Mac. Will this work with SDRPlay software?
There are two ways to get your RSP running on a Mac. Use Parallels or VMWare as discussed above or you can run directly on the Mac with Cubic SDR. This is not as nice as SDRUno or SDR-Console but some users like it. Look in the SDRPlay downloads section
https://www.sdrplay.com/downloads/ There are lots of Youtube videos showing CubicSDR in action.