1) He should be ok to go out and relieve himself in those temps. No need to use the puppy pads any longer then you need to. They will make it harder to house train him. Just keep an eye on him and keep it short.
2) Yes he will need a coat when it's -15C or so. And maybe some shoes or foot wax stuff if your neighborhood lays a lot of sidewalk salt. He should
be fine to walk in the cold, usually they will let you know by limping or flat out stopping. You can also feel his ears, they will get cold the quickest and it's the best way to know if it's just too chilly. -25C is probably too cold for a bulldog.
4) I think they're fine to walk in the rain, just give their ears a wipe when you get home. You could always stuff some cotton balls in them! Lol