At the gate, the gate agent will call passengers up without seat assignments and give them a seat. It might be in Economy Plus and it might be in normal economy (no show, someone who buys up to E+, etc).

As to your other question, not having a seat assignment should not have an impact on whether you are involuntarily denied boarding. If I recall correctly, if the gate agent follows procedure, the passengers denied boarding will be based on booking category and ticket price. However, there have been tales of the gate agents doing the easiest and quickest thing and bumping someone without a seat.

If the moderator moves this to the UA/CO forum someone will provide an exact answer to the second question.