I would call his wife. Tell her you'd like to make a quick visit, but wanted to be sure not to offend her or add stress or drama. Then, if she says she wouldn't mind you visiting, make sure she'll be there when you go, and be brief in your visit. If she says shed feel more comfortable if you'd stay away, ask if sending a card would be alright.
I dont know. There is a fine line between ok and not ok. I think that if you are in good terms with him AND his wife, and if you are close to both of them and they are ok with you coming there, then i think it would be ok. I would just make sure its ok with them first.
I agree with Cobra, above all make sure the wife understands you are not a threat and you mean no disrespect to her. Maybe a small gift of flowers or a card for her would help show your intentions are above board.
That would depend on the kind of relationship you and your ex has had. Also the relationship between you and his wife. Since you and he share children, hopefully you've had at least a very cordial relationship with both if them.
No. I don't think it would be disrespectful. In the same breath be careful not to offend anyone, I would at least call his wife, his mother, or someone close to the both of them and let them be aware that you wish to visit. It's a shame that people won't just be grown ups about important situations like that, the man could pass away and then you would have that on your conscience, that you did'nt at lease attempt to see him one last time. Go see him!