Sinhala people pronounce both ත and ථ as a dental plosive - th. The latter slightly extended form as ttha.
ත - t̪a or tha (unaspirated soft tha)
ථ - t̪ʰa or ttha (aspirated soft ttha)
The word ස්තවිර sounds exactly as the transliterate sthavira. The ත is pronounced t̪a (dental) or tha. The Pali word is Ana[t̪a] or Ana[tha] when pronounced correctly - Ana[ttā] is incorrect spelling.
The word Anattā has a striking resemblance with Sanskrit Ānatman (no-self). Had they taken the dental plosive th into consideration the mistake would have been avoided. It shows what the translators had taken into consideration at the time.
Since you were wrong in the first instance, all reasoning that follows require a revision.
That is correct. Although originally you said.
It seems whilst digging into the history you have inadvertently come across the old Sinhala word that has a corresponding meaning in Pali. So you were wrong from the beginning.