I think that the 90 minutes vs. 120 minutes thing can be filed under "network demands." Although I don't have my copy of "The ColumboPhile" handy, I seem to remember the TV network asking them to produce some longer episodes.

As far as the 3-episode Season 6, if I recall correctly, Peter Falk may have wanted to end the series after Season 5, and the network really had to schmooze him into doing any more episodes at all... which he eventually agreed to do, thank goodness!