Can't login to System Preferences only and other issues

I've been getting random login popup/prompts that my Mac (MacBookPro) "Wants to make changes" for months now. It's because of a Searchhelper phishing virus that periodically creates a profile on my computer, adds the Flash Player plugin to my System Preferences dashboard, and an add-on extension in Chrome.

So today, I went to make changes in my System Preferences dashboard and, of course, it asked me for my username and password. The fields were blank, which was strange since my username is usually already there. I proceeded to enter my username and password here, and it was rejected as "incorrect." I then tried to unlock user info, modify user, access advanced settings, etc., without success—same thing, my username and password is apparently incorrect.

I then tried changing my password in safe mode, but that didn't help. I am able to login my computer fine, just like before, but I still cannot modify any system preferences. Also, I wish I'd noticed before, but I don't have a separate administrator user account on my Mac to login with. The account I usually log in with is listed as a "standard" user. There is no administrator user listed.

So, I'm reaching out to ask:

1.) How I gain access to administrator features in system preferences when login doesn't work, I can't add a user, and there is no other administrator account?

2.) How I add an administrator account for future issues when the plus and minus signs are inaccessible?

3.) How to remove Searchhelper virus for good. I've tried everything short of restoring my computer to factory settings and removing Chrome.

Any insight/help with these 3 issues is appreciated. Thanks.