1) is it possible to have user management in frontend for my customer who dont have backend access ?
To an extent, yes. If they're a CB Moderator they can edit profiles from frontend if profile edit for moderators is enabled in CB > Configuration > Moderation. As for creating new users they'd have to log out first.
2) is it possible to have paid subscriptions (ex: 20$ for first year, 10$ for others years after) with a paypal link.
CBSubs can provide subscriptions and supports PayPal. Not sure what you mean by with a PayPal link.
3) is it possible to display notifications for many event of the website ? (ex: other user changed his email adress, contact form has been spend, ...)
CB Activity and CB Auto Actions can log and display various activity on a users activity stream. CB Auto Actions with UddeIM can be used to send PMS notifications. CB Auto Actions by it self can be used to send email notifications. As for what activity is monitored and notified about depends on if a CB trigger is available (yes for email address and no for contact form).
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