is the sciant and the last syllables of nesciunt the only iambs of this verse?
Yes. I'm not very familiar with Plautine meter, but if this is irregular, I think the only irregularity, measured against Greek iambic trimeter, would be the long syllable sint
. In Greek iambic trimeter, the first syllable of each metron (i.e., the first, fifth and ninth syllables) is can be long or short (anceps), but generally the third syllable of each metron (the third, seventh and eleventh syllables) is generally short.