I think that it is impossible to tell the odds on FT.

The Finnair procedure is fairly standard. If seats free earlier on, your wife will be reissued a boarding pass wit ha seat. If not, when boarding starts, they will call for VDB at the gate. (Unfortunately, they are not very pro-active, and it tends to be missed/ignored.) Then depending if there are no-shows and/or volunteers, your wife will get a seat at the gate closing, or will be IVDB.

I would have a list of acceptable reroutes ready to negotiate in that later unfortunate case.

My anecdotal experience: I volunteered twice this year. The first time, I got in anyway because there were no-shows. The second time, I got in anyway because the SBYers group did not want to split. And late last year, I got pro-actively involuntarily rerouted the previous evening to avoid IVDB.