installing email certificate, plus some noob questions

i’m new in encrypting messages. I’ve read the support thread, but have a few questions.

  1. i received an p7s file and installed keys and certificate into the system (Mac OS). If someone will download p7s file from the link, which was in email from Comodo, does that mean he will be able to encrypt and decrypt my email?

  2. How to install keys and certificate from computer (Mac Os) to iphone, to have possibility to encrypt and decrypt mails from it. (there was a guide to install it by exporting certificate/keys and sending it to email, than open it on iphone. But is it secure?)

  3. When 1year certificate will expire, will i be able to create new one and will i be able to read old messages with new certificate.

Please answer this questions here, i already scaned support thread and other forums, and didn’t find answers.