Unfortunately, with Sitecore 8.X series you can't simply go through each configuration file and comment out or delete definitions for the above indexes, because there are many xDB dependencies (defined in configuration files) that don't respect the Xdb.Enabled setting.
Therefore, to disable xDB related search indexes you must remove configuration files with ...
You should be able to apply a patch like the following:
<?xml version="1.0" encoding="utf-8" ?>
In CMS-Only mode you have zero possibilities to achieve your requirements as you cannot use EXM module (which is specifically designed for these kind of scenarios) and missing xDB.
I would recommend three approaches:
If client already has Dynamics 365 or MS CRM on prem, you can built easily integration with custom save action in WFFM and save contact or ...
As you don't have xDB nor EXM it will be difficult (or impossible) with ootb WFFM. I see 2 options:
fully custom form
custom save action to send the mail: you could create your own save action to send the mail with the activation link. You have a few options to generate the link: use the formID, the email (with some extra's or I can activate anyone), a ...