I think I've made a few errors in soldering together the perfboard that the Attiny sits on. At the moment, it doesn't appear that I've wrongly soldered the SDA/SCL leads, but one or two input pins have definitely been wired the wrong way, so there is a chance that being off to the left or to the right by one solder spot may have created connections that aren't supposed to be there and that also affect the SDA and SCL lines.
But just in principle, so I will be able to eliminate that error source - what is the right way to send several different bytes in a row from an Attiny(84) slave to an Atmega master? How does the code have to look?
With regards to my Attiny sketch, is this the right way:
TinyWireS.send(sendData[i]);
i++;
if (i == 8) i = 0;
or this?
for (int i = 0; i <= 8; i++) TinyWireS.send(sendData[i]);