What do I think? That you're trying too hard to find something which isn't there. Come on, it can't come from pre + g. First, what this "g" stands for? What does it mean? Second – how else do you explain Polish cognates (naprężyć to name one) which all have NASAL vowel here? And Czech cognates (zapřáhnout) which show Czech typical reflex (á) for nasal vowels (which, surprisingly to you, did exist in the Proto-Slavic stage)? Do you really think that nasal vowels can just magically "appear" at one stage and disappear at other? I should remind you that nasal vowels cross-linguistically come from the sequence vowel+nasal consonant (usually either [m] or [n]). However I feel like I'm talking to the wind because you seem to not listen to reason.