@jtrkwok The hacker almost certainly will want to maintain some type of access (backdoor) for further malicious activity. This is usually in the form of a malicious file that can be used to upload further malicious content without the need of exploiting anything again – they just use the malicious backdoor file.
I would recommend trying to download the Sucuri plugin through the wp-admin dashboard and see if it gives you the same error. If it does, you can do this work around:
1) download the plugin’s .zip file here
2) using FTP or File Manager, upload the sucuri-scanner.1.8.21.zip file to your /wp-content/plugins/ directory
3) unzip sucuri-scanner.1.8.21.zip, it should unpack and create /wp-content/plugins/sucuri-scanner/ with all the plugin files.
4) go to your wp-admin, navigate to /wp-admin/plugins.php and the Sucuri plugin should show up and be available for activation.
I mention this plugin before replacing wp-admin or wp-includes as the plugin can compare your core WP files (wp-includes, wp-admin, etc) against the files on the WP repository corresponding to the version of WP you use.
It will list the files under “WordPress Integrity” if any have modifications like injected code. You will want to review the flagged files to be sure it was not anything you or a legitimate plugin added and is needed by your website. Nevertheless, be sure to backup before changing anything.
If there are just a handful of core WP files that were infected, then you can replace them by hand from the available files for all versions of WP here
And yes I think the issue you faced 2 months ago is related as that is not normal behavior for WordPress and the email accounts issue may also be due to malware as some malware will create email accounts just using a PHP file they uploaded and not ever logging into your cPanel like is normally done.
They aren’t trying to rip off your visitors, but rather just use your website’s good standing (e.g not blacklisted) and just spam out phishing emails that contain a link to the phishing pages they set up on your legit website. Any of the details submitted by the victim get sent to the hacker via email (sometimes they use different methods) and they just keep sending victims until you secure your website or until it is burned (blacklisted to the point it is no longer useful).
Hope this helps.