As long as you follow the guidelines for protecting and using your phone, your data is well protected.
For additional protection, sMessenger is designed in such a way that, even if the normal protection of the data on your phone fails, all messages, notes and contacts saved in sMessenger stay protected.
If you are afraid of Google, you can switch off almost all Google-related apps: sMessenger can perfectly run on an Android-phone with all Google-related code removed.
The answer depends on your needs:
The short answer is No; a normal sim-card from a normal provider will do the job.
There is no need to use a VPN-connection: sMessenger has a build-in VPN-connection.
If You want to obfuscate your access to the servers of sMessenger, feel free to route the internet trafic over the TOR-network.
The mailserver of sMessenger will accept incomming mail from all domains and is not using whitelists and blacklists.
However, when the mailserver from another domain refuses to accept mail from sMessenger mailserver, communication might prove to be impossible. Note that this has nothing to do with sMessenger being safe or not.
We at sMessenger believe tools for protecting privacy should be affordable for as much people as possible, limiting communicating to certain domains does not fit in this picture.