Are there some differences between 信 and 讯？
The difference between 信 and 讯 is explained here Differences: 信息 vs 讯息?

讯: message; a specific piece of information

信: letter (contains information in general)



通风报信（why not 讯？）

Type in 通风报 in the search engine, the first page will be filled with 通风报讯 entries. Obviously, some people use 信 and 讯 interchangeably and made 通风报信 a somewhat accepted variant of 通风报讯 which is the standard one
通风报讯 (Whistleblowing) is more appropriate than 通风报信 because the act of 通风报讯 is more like sending a message than giving a report

新华社讯（why not 信？）

讯 in 新华社讯 implies there are specific pieces of information (news items).
新华社信 implies 'a letter from 新华社' -- like an editorial
More examples:
通信 = exchange letters (互通书信)
发信 = send a letter (发出书信)
通讯 = exchange message (互通讯息)
发讯 = send a message (发出讯息)
发讯号 = send a signal (发出讯号)