I've been researching DUI impact on immigration to Canada and here are my findings.
1. Anybody who had DUI in any country on Earth are inadmissible to Canada for 10 years after the sentence is fully served. Trying or entering Canada with recent DUI is criminal offence under Canadian statues. In reality there are cases when all 4 people in a car told border officer about their DUI and still we let into Canada.
2. After 10 years from DUI one is deemed rehabilitated and good to go, like nothing ever happened.
3. After 5 years one could send an Application for Criminal Rehabilitation and after some time (that could be from one day to a year) could be deemed rehabilitated and good to go.
4. Any application for permanent residence or any visa for that matter by the person who had DUI and is not rehabilitated should be automatically denied on the grounds of inadmissibility.
5. When applying for permanent residence, certificate of rehabilitation should be obtained prior to filing an application, however there are rare cases when application was put on hold until rehabilitation certificate was obtained.
6. At least one lawyer advised me to send both application for permanent residence and application for criminal rehabilitation together and asking for "compassion" with regards to rehab arguing that having immigrant application on file could move things faster. There is a possibility in this, but I doubt this could work out for generic federal application (I am Quebec nominee).
Finally, pertaining to the original situation in this thread. There is a chance of obtaining "temporary permit" prior to being eligible for rehabilitation (i.e. prior to 5 years). It is to be seen if this permit could be used in connection with an application for permanent residence, but a good lawyer perhaps could manage that arguing that application for criminal rehabilitation will be comming.
Regardless, once application is denied, the only easy way to "get it back" is thru CIC administrative appeal, but in the case in this thread, this appeal would need "temporary permit" and unless it could be obtained very fast, because of very strick limitations when the case could be appealed, I think the chances are very small and thus most conservative lawyers should refuse the case.
And thus the moral of the story - If criminal record is anything but clean (including traffic violations accociated with traffic accidents) the first thing to do when filing federal application is to obtain rehab letter or retain a lawyer.
P.S. On the subject of Quebec and other provicial nominees. Criminal checks are beyond provinces jurisdiction, thus it is possible to obtain provincial nomination prior to resolving criminal issues. Yet being provincial nominee in no way guarantees immigration to Canada - that is why exactly there is federal level. However, provincial nomination could help and could be obtained in paralel to cleaning out criminal record.