My question is if I don’t want to go through all the trouble of changing the site back to a regular WordPress site, can I just treat and use the site as a regular WordPress site?
I think you might be overestimating the trouble. If it’s a multisite with just one main site then simply killing multisite bits from config will de-multisite it. Or at least that’s what I remember, it has been a while since I did this.
And since it’s a multisite and I only have admin power and not super admin, to install plugins I would have to either ask for access to the super admin account or have the person install the plugins for me, right?
Correct.
Do I just download Filezilla to grab the files and change them and upload them back? Or is there a better way to do this? And would I need super admin power to do this and what additional information do I need to set up the Filezilla besides the site url, my admin account and password?
Using FTP (but really, you should at least use SFTP for security) is pretty common. However it is in no way connected to WP installation. Any account for it would have to be set up on server level and will be unrelated to WP accounts.