Phonotactic processes during Syncope in Pre-IE

I'm really glad that I've finally made a summary pdf of my Pre-IE theory because it's helping me focus my concentration on smaller details now. I was pondering again these past few days about phonotactics, syllable structure and all that jazz in relation to the rule of Syncope in Pre-Indo-European (PIE), trying to tease out more revelations about the details.

One thing I just noticed is that there seems to be a kind of symmetry presenting itself between two different phonotactically motivated processes happening at the same time. "What does your prattle mean?" you ask? Well, I mean that, as the supershort schwas were being deleted in the late Mid IE, this must have put incredible pressure on syllable structure such that special countermeasures were needed to avoid hard-to-pronounce consonant clusters, especially at the beginning of a word where the consonants tended to clump due to the previous penultimate accentuation of Mid IE. So, it seems to me that while Metathesis occurred in positions after the stress accent to remedy awkward syllabification, a-Epenthesis (the left-to-right insertion of *a) occurred in positions *before* the stress accent. I notice that Mongolian also has a similar process of left-to-right schwa epenthesis however this seems to occur to resolve clustering in the coda not the onset.

So this symmetry of phonotactic processes in Pre-IE seems to exist... or at least so I think so far... but should I believe this or am I seeing a mirage?

This train of thought may be opening up another can of worms for me. a-Epenthesis only seems to be attested in noun and adjective stems. When I ponder on how verb stems might have resolved word-initial consonant clusters however, a-Epenthesis doesn't seem to do the trick based on the evidence in later Proto-IE. Let's say we have an imaginary verb in Mid IE like **raténa- which is for the purposes of intellectual inquiry a well-formed verb root at this stage (n.b. the same structure seen in *sad̰éxa-, the presumed antecedent of *steh₂- "stand"). Everything's good so far... until Syncope happens and threatens an illegal stem form **rten- that violates sonority hierarchy. This means that if a-Epenthesis wasn't triggered in verb stems at all, something must have been triggered to avoid this very ugly scenario. Might Metathesis have operated in cases like this too, thereby producing **tren-? Is there something that I'm missing? This problem has been driving me nuts for a while now because I can't for the life of me think of a possible example showing this kind of metathesis in the onset of a pre-IE verb stem.