Is then a conjunction

Answer 1

It can be, rarely. More often it is an adverb.

In the sentence "He picked up a suitcase, then left" the word then is substituting for the conjunction "and" while alsogiving adverbial information. This is tricky but apparently acceptable usage.

Answer 2

"Then" is not a conjunction; rather, it is an adverb used to indicate time or sequence. It can be used to show a progression of events in a sentence.