This could also be explained as simply information that hadn't been provided to us earlier, couldn't it? I haven't seen the pre-Barnabas episodes since they aired on the Sci-Fi channel, but I don't think much information about Willie's background was ever provided. The emphasis was on his actions at Collinsport. It could be that he had done odd jobs in the past, and that he had then described this background "off-stage" to Barnabas or the Collins family. Although you may be right that this was probably "retroactive" on the part of the DS writers, I prefer to think that it harmonizes with what little information we had been given. In plays, "off-stage" information is common.