I managed to pick up the Sky trophy on its own from my local CEX for 50p (though I only picked it up because I already had a Sky vehicle), and I think they had the Sea one for 50p as well, so if you already own the vehicles that's an option, otherwise these links are the best way to play the extra tracks and modes. I was wondering why the Land trophy was so expensive in CEX compared to the others (£15 just for the trophy!), but that definitely explains it.
Link to sea pack https://www.game.co.uk/en/skylanders-superchargers-racing-pack-sea-751647
The Air Racing Pack with trophy is available from Amazon Warehourse for £3.43 currently (or £4.97 new) https://www.amazon.co.uk/Skylanders-SuperChargers-Racing-Action-Astroblast/dp/B013HJ7AQW/ It's a shame the Earth pack is so rare (I think it was a Toys R Us exclusive in the UK)
Not with figures, but true with Superchargers vehicles: in Imaginators you put them on once and they're added to your collection and you don't need them again. This wasn't the case for the Superchargers game.
That's not the case with figures i think, only with items that unlock new levels. You need to have a figure on the portal to use it as playable character.