Couple of thoughts:
1. Launching - Penzance harbour by the sailing club (Google plus code “4FC9+4J8 Penzance”) When we went (2014 pre kids) we simply parked the car with trailer attached in the harbour car park and all was fine. Since then, a fence has been erected for boat and trailer storage which I assume is associated to the sailing club. I don’t know if my assumption is correct but you might be able to agree a fee with them.
2. Camping - we camped on Bryher at the campsite there. Although they will likely show as being full on the dates you end up going (if it’s this year) I’d suggest calling as they are very accommodating and will find you some grass to pitch. I’ve not camped on the other islands so can’t comment. If you do end up heading for Bryher, passage plan on having a minimum 1.4m of water to access the Bryher/Tresco channel from the south.
Definitely do it though - it’s been our best trip by far and has caused us to return to Bryher as a family. We only got back from another visit a week ago!