Yuri is formal, Yura is a diminutive/familiar form of the same name - for friends and family. They're interchangeable.
It's sad that they got caught. Never plan delicate things over the phone, especially in a repressive country on war footing (I think that in Russia they record absolutely every phone call, down to the last consonant). Apps aren't safe if the device isn't safe, so apps aren't for planning either - they're for chat and establishing initial contact before switching to a more secure method, or following the activity of publicly known folks who presumably take great care.
I've never lived in such a country, but regardless, when I still was a bad anarchist (TM), I left my phone home when going to meetings, and a friendly organizer went around with a bucket before any talk occurred - to gather the phones of those who carried them, and take them to another room.