Hey Ska,
Been a while since I've checked up on your tank. Sorry to hear bout all your aquarium troubles! I'm glad you haven't given up yet. Being a saltie - there's tons of rewards but also some real tough lessons to learn both emotionally and financially for a 'light' hobby.
In terms of the choice of macros, especially caulerpa - one has to be careful in chosing the varieties. I've found any for of caulerpa racemosa (espcially any form of grape caulerpa -var. pelata) and caulerpa prolifera (ex: knife caulerpa) to be especially aggressive. Others include sawband caulerpa and sometimes C. Mexicana.
In your tank's best interest, I would very strongly suggest you get rid of all your live rock and sand that was covered by your grape caulerpa. From experience both Grape caulerpa and Knife caulerpa can re-grow from just a few cells (root, stem, any sort) clinging in a fissure in your live rock. So for months after you think you've cleaned it all out, suddenly you may experience a re-birth/ explosion of this nuisance macro. I know it seems like such a waste of money/ live rock but think of it this way, by throwing them out now, you're saving yourself a ton of future grief and heart ache. You can always re-use your old live rock after you've let it dry thoroughly for about 3+ months.
As for having to spend money on new live rock - look at it as if it's a fresh start. To save some money, you can buy a third of the amount you need live and the other 2/3s dry rock/ man made which will be seeded by the new live rock.
In terms of suggestions for future macros, the safe forms of caulerpa in my experience are Caulerpa Paspaloides (Palm Caulerpa) and Caulerpa Cupressoides (Cactus Caulerpa). Both are comparatively slow growing and easy to manage. In addition, they look nicer than most other caulerpas
.