Greek double-sigma / double-tau

Hello all,

Concerning the double-sigma/double-tau in Old Greek (for example, thalassa
versus thalatta):

Which was the original form, ss or tt?

What was the IE "root sound"?

In which dialects, exactly, and at what time(s) during the course of "old
Greek" did it change from one to the other?

Is there any particular reason for the change(s), or was it just a
run-of-the-mill sound shift?

Is there any easy way for someone actively studying old Greek morphology
and syntax (me :) to tell when the change has occurred in a word, just by
looking at a word and not knowing anything about the word?

Were the phonemes of the double-sigma and double-tau rearticulated (e.g.,
"two unvoiced dental stops 'right in a row'"), or was the first phoneme in
each said and then "held on to". (I don't know the term for that,
obviously, I'm sure you can figure out what I mean.)

Concerning specifically the word thalassa/thalatta, was that a native word
before the Greeks moved in? (If so, did it mean "sea" previously as
well? And what is the oldest known form of it written?) I haven't been
able to find a cognate in other languages is why I ask.

TiA,
Lisa