The sungoi is most likely due to the Tokyo nasal g accent. In Tokyo, oftentimes when there is a "g-" sound in the middle of a word, they'll add in a subtle "n" right before it. They'll probably still write it as sugoi because that's what they're saying, just with an accent. This i have learned from my time on WaniKani because one of the people they use for vocab pronunciation has the tokyo nasal g thing going on and it really confused me until someone explained it.
For the lengthening, it's pretty easily compared to english. when we want to emphasize a word, we stress the syllable that already has the most stress on it. try it with "totally" or "extraordinary." In japanese, they don't really use strong and soft syllables like we do, but changes in pitch. I'd compare raising pitch to beginning a strong syllable, so modifications for emphasis go where pitch increases.
Hope this helped! :)