WreckingBANG

Signal uses 2% a day and Element 1%. I do not really think this is significant.

Depends on the App but generally no.

It depends for what Apps you need them. Often Apps like Signal or Element support getting Notifications via a persistent Server connection or asking every so often for Notifications. Even tho i could not find any source, i found that this works with Whatsapp(sadly have to use it) too.