Messaging apps compress your photos anyway, usually badly and without asking. Shrinking them yourself first means you decide how they look.
Most messaging apps re-encode photos on the way through, which is why pictures arrive looking worse than they left. Sending an already sensible file gives the app less to butcher.
Width matters more than quality. A 1600 pixel wide photo is sharp on any phone and is a fraction of the size of the 4000 pixel original.
If the picture genuinely needs to arrive untouched, send it as a document or file attachment instead of a photo. Most apps skip their compression for those.
Less than the messaging app would have made it look, in most cases.
Yes, add as many as you like and they all get the same settings.
No.