Well, the restitution of the first vowel in the perfect for the forms with the heavy suffixes ־תֶם and ־תֶן happens colloquially with all verbs, reflecting the Middle Hebrew shifting of the stress from כְּתַבְתֶּ֫ם to כָתַ֫בְתֶּם. The speaker here seems to use the mixed form כָתַבְתֶּ֫ם with ultimate stress, but full vowel in the first syllable. Traditional הֱיִיתֶם is irregular inasmuch הֵא usually takes a חֲטַף פַּתַח like הֲלַכְתֶּם. PS. The infinitive, traditionally לִהְיוֹת [lihyot] is difficult to say inside the phonotactical rules of Modern Hebrew and is therefore colloquially replaced by forms like [lihiyot] or [liyo].