You can send your photos before or after. I will not start drawing before we both agree and until the purchase is made.
You can do it by attaching the photo through a private message.

Once the pencil drawing is done and approved by the client, I will apply fixer to avoid smudging.
Depending on the size, I will send it inside an envelope between cardboards to avoid bending.
Larger sizes will be shipped in tubes.

Yes. There is no problem with that.
The main thing to keep in mind is that they must have similar lighting and that they are taken at similar distances.
That is, a photo under the sunbeam and a photo taken at night with a flash, will not look good together.
A selfie and a photo taken from a distance will not look good either.
You can send me the photos by private message and I will evaluate if they are combinable.