otherwise i'm not sure why it's not being set in 1967 or for that matter the present. it's got to be a stylistic decision.
6. Personally, I think setting it circa 1970 is a brilliant idea. If somebody showed up today and told anyone "I'm your hitherto unknown cousin from England come to stay" the first thing they'd do is do a google search. Which in Barnabas' case would turn up nothing. Nada. Zip. Same thing for a credit score. But in 1970 someone might be able to pull it off.
And just because we only know about Angelique, that doesn't necessarily mean she was the only one he slept with and then dropped flat.