Help with oil sump gasket

1. Could the leak be caused by a deformation of the metal sump itself which and if so could I just hammer itself into place or need a new sump? Could anything else be causing the leak?

Yes, most definitely. If you can detect exactly where the leak(s) is/are coming from, and can ensure the leak is off the oil pan, then it's either the pan being out of shape, the gasket going bad, or a combination of both. Until you can absolutely nail it down to being the oil pan gasket (don't assume, ensure that's what it is), don't do any work on the car. By taking the pan off with it not leaking, you'd just be causing yourself extra work.

2. When I attempt the repair it could be I have the wrong gasket part or find the sump was damaged in which case I’ll want to stick the old part back on since I don’t have my own garage/workshop. Will it be ok to just bolt the old part back on though I know the leak will still be there?

If you are worried about it leaking, use a Permatex product called The Right Stuff. It WILL seal it without issue. It is made to seal on tough jobs.

3. if I do the work outside and the sump is removed for any amount of time might dust and dirt be getting into the engine from below and is this anything to worry about or could I even leave car outside with sump removed for some days without issue?

If you need to leave the car sit for a few days, put the sump back on loosely to cover the bottom end of the engine. This is just to cover it up to keep the majority of anything out. Unless you're in a clean room, you're not going to keep all of the contaminants out. This is nothing really to worry about. Any contaminants which get on the engine will just be on the exterior parts of things. Right after start up with fresh oil, the only thing any contaminants will touch is the oil pump, then get trapped in the filter before the oil goes through any working parts of the engine (like bearing journals and what not). Just do your best to keep things clean and it will be fine.