How soon after they attempt a mechanical defusing would they detonate?
Well, the fuze would detonate pretty much instantly during this treatment.
Here is a simplified drawing of one of such fuzes (Usually you also have a lot of stuff there for making sure it is not that easily defuzed):
The trick is to get as much distance as possible between fuze and bomb in a very short time. That is achieved by those rocket clamps. You have essentially just milliseconds before the bolt inside the fuze triggers it by hitting the detonator. About 20 cm distance is already enough to make sure the detonator charge can't properly trigger the bomb anymore.
With the slightly less risky mechanical impact fuzes, the UXO teams use strong hydraulics for simply pulling the fuze out of its thread, should it be impossible to simply unscrew them.
This PDF file explains it a bit more in detail, the BDFWT is the organization of all UXO disarmers in Germany:
http://bdfwt.de/wp-content/uploads/2014/10/13_lzzdr_kw.pdf
The important rules for all bomb disposal workers when such a fuze is encountered, from this file:
- The safety of the disposal team is more important than economic interests of the population.
- The bomb disposer should only stay as short as possible and as long as necessary near the bomb
- Only have as much personal near the bomb as necessary
- Treat all bombs with the highest respect, bomb #999 just like bomb #1
- Plan the disposal with the highest care
- Prepare your tools well, so that nothing is missing and has to be reordered during a disposal
- If you are not sure, study the datasheets and disposal manuals or ask more knowledgeable coworkers.
- Never play the hero, you have only one life.
- Don't begin the defusing before all other people have left the danger zone.
- Never let other people (politicians, journalists, property owners) put you under pressure.
- You choose the radius of the evacuation zone and suggest it to the police. Should the police change the radius, let it be documented.
- Remote defusing is always better than manual defusing.
- Never move the bomb before it is defused. If it was already moved, respect the wait phases after each movement.
- Only issue press releases after defusing the bomb.
- Never lead uninvolved people to an armed bomb
- The disposer on site decides about the procedures. He should make use of the technical state of the art.
The aircraft at the nearby Düsseldorf airport are staying on the ground during the bomb disposal. The defusing was delayed to 14:00 because there are still people inside the evacuation zone.
For the statistic: This is the 89th UXO found in Düsseldorf since 1995 and the third one with a chemical delay fuze.
---------- Post added at 02:12 PM ---------- Previous post was at 01:40 PM ----------
The UXO team has started to defuse the bomb.
---------- Post added at 02:33 PM ---------- Previous post was at 02:12 PM ----------
For the record: The leading bomb disposal worker is named Jost Leisten (Nice to see a name there before something bad happens). He estimated the defusing to take about 90 minutes.
Putting a face to the name, the man with the fuze is Jost Leisten. He disarmed quite many bombs already, there are dozens of cases in this state of Germany where he disarmed UXOs since 2010 (That year is the earliest newspaper article with his name I can find, I am sure he started this job already earlier).
---------- Post added at 02:54 PM ---------- Previous post was at 02:33 PM ----------
And: The bomb is disarmed, after just one hour.

---------- Post added at 03:20 PM ---------- Previous post was at 02:54 PM ----------
The evacuation is now over, the people are brought back from the shelters to their homes. Trains are already resuming normal schedules.
Only one plane had to be redirected to Cologne-Bonn while the airport was closed.
---------- Post added at 03:25 PM ---------- Previous post was at 03:20 PM ----------
The disarmed bomb before transporting it away for disposal: