[Avodah] Targumim from Sinai

[Simon Montagu]

RSBA reminded me in an Areivim thread that the Targumim are from
Sinai. This surprised me at first, because my memory was that at most
they are described as "beRuahh hakodesh", but I was wrong: the Taz on
SA OH 285 SK 2 explicitly says "hatargum yesh lo ma`ala she-nittan
besinai".

I have a number of questions on this.

1) Is there an earlier source?
2) How can this be reconciled with the gemara in Megilla? I can
imagine one possibility: the targum was given as part of TBSP and
Onkelos was the first to write it down. IMHO this fits well with the
phrasing "amaro mipi RE veRY", but does anyone say so?
3) Does it include Yonatan BU and Yerushalmi, or only Onkelos?