GO debuts mythicals in paid research during GO Fest first then roll 'em out for free for all players later. You shouldn't need to pay for Marshadow.
You could snag Magearna with the QR code but yes, if you haven't installed Bank before you can't download it now.
If someone touch traded you your missing mythicals, you'd unlock the Original Magearna gift.
Ahh interesting! I'd taken a long break from GO as well, so didn't realise I'd have a chance with Marshadow at a later date as well.
Thanks for the info! :)