Using LTE variant with non-Multisim Smartphone

Hello Galaxy Watch fans, for an elderly person I am considering the following use case. They mostly want to use the smart watch for displaying Whatsapp texts, receiving calls etc. In cases of emergencies, they want to be able to send out an emergency call even if the phone is not within distance.

Their phone contract does not offer an additional SIM, so the watch (LTE?) would need its own contract with a different number.

I am wondering how problematic such a mixed LTE/phone usage would be.

For example:

• how would WhatsApp work in such a scenario (does it run on the phone or the watch? can it still receive WhatsApp messages from the phone)
• can the watch deal with outgoing calls going via the phone if its in reach but via the eSIM if not?
• Assuming the watch can display WhatsApp messages that the phone receives, how does the communication between Watch and Phone work? Does it use bluetooth or wifi or mobile data?

Would be great if you could help me out with this, its hard to derive from the manual.