Could a short walk (or similar exercise) lead to cardiac arrest ?

Probably no more likely to result in an arrest than sitting down. Of course overall, taking exercise will reduce your cardiac risk.

depends how fat and unhealthy you are.............

If you are very unfit or unhealthy and obese it would probably cause you to exert yourself, so for some, it technically could...

No. because there wont be significant increase in your heart activity. there will be enough supply of oxygen to your heart
If you feel your heart beat or breathing rate has increase, its better to take a rest and continue with the walk after fully recovered

Not unless you already have major heart problems.....It is best to get the advice of your Doctor always.

If you have extremely blocked arteries, yes it could.

The effort could be too much, and the arteries might spasm when they could not get enough oxygen, further contracting, and causing a heart attack, though maybe not cardiac arrest.

Happened to me.

It depends on existing comorbid factors.

Impossible to answer.

Yes, of course it can.

You should remain sitting on your sofa and eat as much food as possible....not to mention lots of cans of beer and coke! Any movement at all could possibly kill you! Arrange for your District Nurse to come in and fit a urethral catheter so that you don't have to get off the sofa to visit the toilet.