The only benefit to DNA testing would be if someone else is a match as a relative, and they have done some tracing, then you have a family grouping to work forward or back from - it's better than the nothing that that we have now. It could give us a lot of erroneous leads, but I'll take anything right now. If there is no serious connection, then it provides nothing and it was a waste of money.
Definitely try asking everyone! You are lucky with respect to the fact that people are still living. I only have one person who knew the great grandfather, and his memories are those of a 5 year old. If he was born in 1918, there could definitely still be people who remember something. Anything you get will be family folklore that will have to be proven, but it will give you directions to look in.