Here are results of manual testing:
1) If user puts & or &amp; in transcripts on youtube with utf-8 encoding - it will be processed as u'\u0026' and u'\u0026'... after parsing by etree.fromstring to unicode.
2) Resulting unicode is saved to srt sjson file to assets via json.dumps which uses utf8 by default encoding.
3) Then browser interpret both & and &amp; as &, so
conclusion is that unescape is not necessary here. 
Thanks. Fixed.