Just to check, When the last vowel is lengthened is that syllable also stressed, as opposed by initial stress?
Well, at least when its lengthened as a way to express "and." In other contexts, a long syllable will not move the stress to the end.
In fact there are only three reasons for a word to be stressed on the last syllable in High Valyrian:
Conjunctive lengthening: taobā "and the boy"
Imperative verb: edrūs! "Sleep!"
Borrowed from another language: buzdar "slave."
Ghiscari, the language of the old Empire of Ghis. As far as I know, it's not a fully developed language, and only serves to provide some unique vocabulary to Astapori Valyrian.
It is unusual to use se with bare nouns. Normally if "and" is going between two nouns, unmarked by adjectives, you use lengthening instead. If you use se it puts strong emphasis on the "and," resulting in something more like "The man and the boy are sweating" or "Not only the man, but also the boy are sweating."