That which you say is correct is certainly what I would have written, and I understand all your examples just fine, thank you.
So in the end, 'θα πήγα στην Ρουμανία' is just incorrect Greek then. I was confused when I read it, but assumed that as it came from a Greek source, it must be right somehow. When I Googled "θα πήγα", I got some 2000 results, which added to my confusion as to the meaning and/or correctness. Looking again, I think they are Ancient Greek, though I haven't checked that properly. Does 'θα πήγα' exist in Ancient Greek? I only have a very vague knowledge of it.
I have looked out my Greek grammar book since posting. Definitely written there that this is correct Greek (a second form of the conditional, it says):
Θα είχα γραμμένος πιά (I would have already written)
(with the accent on πιά, though I wouldn't put it, I agree). The book includes a lot of rather old Greek too - so I guess this form must be archaic. As I say, it wouldn't have occurred to me to write the Greek in that way, I would write 'Θα είχα γράψει κιόλας'
Many thanks.