Hi all, thought I'd give a quick update. We haven't lost any more chicks since the last update and they all seem to be thriving!
We got the vet out who was extremely helpful. He thought the setup looked fine and that we were doing all the right things, but had some things to try regardless. Firstly we got rid of the pine shavings. There is a chance that some or all of the chicks were eating these and essentially starving to death as one of you mentioned! We just started using paper towel and replacing it at least twice a day. Not the best environmentally, but we were willing to try anything!
We also replaced the heat lamp with a brooder plate. It's coming into summer here, and although we were monitoring the temps and watching behaviours, it's possible the best lamp was just to hot...
In either case, the remaining three are doing very well, and we introduced 3 more friends which are also going well (just a week apart in age).
They're again to everyone who offered advice and kind words!
Now here's to hoping we have mostly pullets! Best guess so far is that we have 3 pullets and one roo in the Houdans, and it's anyone's guess for the Araucanas. Three of the houdans have had extremely pronounced tail feathers from earlier than one week old, and one still has a fuzzy round butt. I realise this isn't super reliable, but I'm hoping that's the case haha.