And here we have the problem with trying to answer questions in 140 characters or fewer.
Guys, I admit I'm doing some educated guessing, but I don't think Mike's answer meant that it was up to the DM whether a warlock's limitations on invocations was based total level or class level. On every other occasion, he's clarified that things based on level (i.e. cantrips) are based on total. He clarified that specifying "paladin spell slot" for divine smite was a mistake. The PHB, as written, does not contain any rules for judging level-dependent effects by "class level." (Well, I guess the multiclass rules for learning spells kind of qualifies, but it's very specifically called out as an exception.)
I think Mike was referring to the "retraining" part of the question which (if one only quickly skims the question) could easily jump out as the primary concern. The PHB doesn't have rules for retraining, except for swapping out spells. I can easily see that being a table-by-table question.
One Invocation is explicitly allowed to be retrained every time you take a warlock level.