take the old phone number and the new one and list them under the same contact name
Instead of having say Peter Vodafone and Peter MTN, you can have just one contact with the 2 numbers
Then his should reflect in whatsapp as well after some time
I would imagine these problems need to be solved for battery issues:
disable background apps from running
Remove unneeded apps
Disable wifi/cellular when not needed
Look at the battery monitoring app to see which apps/service uses the most battery
For wifi issues - readd the network
For...