I live in Cornwall, I hate these sort of threads which inevitably turn into Cornish bashing / tourist bashing threads.
People in Cornwall are understandably nervous about holidaymakers visiting again, as are other tourist hot spots all around the country. The worry about hospitals isn’t that non locals shouldn’t use them, but they are smaller and have less resources, in combination with a (usually) higher percentage population of elderly people. In the summer, even without covid the acute hospital and minor injuries unit cannot cope and are always non-black alert.
I have never heard of locals vandalising a tourist’s car, however having visited st Ives in high season it is awful. Chocka block on the tiny streets with people - social distancing will be difficult, if not impossible, small streets and one way systems which are easy to get stuck in, with people walking all over the street you are trying to navigate.... nowhere to park except up a huge hill - great to walk all your beach kit back up the hill..... st Ives is beautiful but it is a victim of its own beauty and is horrible in high season. Porthcurno also gets extremely busy. If you are set on St Ives then I echo previous suggestions of the train from Lelant or St Erth.
OP - are you set on that area? There are many other lovely places which are not as cramped and difficult to navigate/park. Newquay gets a bad name but it has beautiful areas outside the town, seven sandy beaches and more parking. Wadebridge is less popular but the area has some lovely beaches nearby and lots of places inland to visit if you like walking/cycling like Bodmin Moor, the Camel trail and Camel Valley Wine (lush tasting sessions)! It all depends what you are looking for.
I have friends who run a B&B and they have said that the date when they can reopen is 4th July providing there isn’t a resurge in covid cases.