Does it mean that the distinction between 'plain' voiceless and aspirated plosives is neutralised word-initially?
Do you mean in a single syllable nucleus? Or across syllable boundaries? Do you have some way of avoiding longer clusters of vowels? (Like inserting glottal stops or whatever in between?)
I did not fully understand your first question but I will try to answer:Every word-initial plosive is aspirate or palatalised.It is never plain voiceless.
Not in single syllable nucleus because syllable structure allows only one vowel in a syllable.
Vowel clusters only appear at syllable boundaries.Example:naiṭec-syllables are na,it and ec.
Why there are no vowel clusters longer than 2 vowels:
If a plosive precedes a vowel,it becomes aspirated by fusing with a vowel.
If a cluster is word-final,the last vowel becomes a schwa and then dissapears.
In other situations,last vowel also becomes a schwa and then dissapears.