Direitos de check-in, overbooking e compensação da UE on-line

I can't tell you about the specifics of your airline, but let me tell you in general how this works.

It is extremely unlikely that you will be denied boarding for overbooking when you check in, either online or at the counter. It virtually always happens at the gate. Airlines do not know until the very last minute whether all the people with tickets will actually show up, even if they are checked in. If someone doesn't show you will be let on board.

You can tell at check-in if you are at risk. If you are given a seat at check-in then you will almost always be let on board. If you are not, then you may be bumped. If that happens staff are explicitly told not to say that you are at risk, to reassure you and tell you everything will be alright, and often it will, but you are definitely at risk if you don't have a seat number. Note that you can be given a boarding pass without a seat number - it will say something like "seat assigned at gate" or "see gate agent". Despite the reassuring wording this definitely means you are at risk of bumping. (Obviously none of this applies to airlines where seats are first come first serve at boarding).

For regulations it makes no difference whether you are bumped at the counter or the gate (though it will virtually always be the gate). You have been denied boarding and are due whatever compensation the law and your contract says.

being denied issuance of a boarding card with a seat number at online check-in phase,

I've never seen or heard of someone being denied a boarding pass at check in.

It it's full, they may offer you compensation to voluntarily take a different flight, but most airlines will still let you check in and give you a boarding pass just without a specific seat. It typically says "Seat assigned at gate" or "see gate agent for seat assignment".

In this case you are expected to show up at the airport and be at the gate at the appointed time.

Airlines want to minimize the cost of overbooking. So they tend to wait until late in the game to estimate how many people will actually show up. Then they are looking for volunteers to give up their seat for compensation. This typically turns into a bit of an auction. They will offer some amount of vouchers and see who goes for it. If they can't find enough volunteers at that price they often increase the offer until the numbers work

Involuntary bumping happens only rarely since it's expensive to the airline so they rather throw a money and specifically vouchers at volunteers. If they do bump, they have rules and a pecking order to select the victims. Keep in mind that a seat assignment and a boarding pass does NOT protect you from being bumped. If their internal rules say "you're it" then "you're it". You can actually be dragged off the plane even after you have already boarded. Case in point: https://en.wikipedia.org/wiki/United_Express_Flight_3411_incident