Do all words belong to conventional truth (Sammuti Sacca)?
All words are like sign posts. The meaning of it is an idea that is in the mind of the person who is speaking it. And the communication will take place if the person who hears it has assigned the same meaning to the word in his/her mind.

Therefore the shared meaning has to be previously agreed upon or communication will not be very effective. As a result it is extremely difficult to communicate an idea (ex: enlightenment/dhamma) when the receiver has no data points to relate to. They become merely words. 

So yes, the words don't have the properties of what they refer to. For example Alan Watts once said that "You can't get wet from the word 'water.'".