How big is the codebase? Obviously you’re facing a rewrite, but I would start by reviewing the logs and codebase, think deeply about how it should work, and isolate the current solution and mitigate the problems while you start building the correct solution.
You need to begin by getting your arms around the biggest problems and begin to address them this week. Start by installing new relic, tackle problems with zeal, and do it first on a staging environment with plenty of testing so you’re not introducing more problems.
while you’re doing this, sort out a proper gitflow based continuous integration for testing, and continuous delivery with deployment automation. Also put everything your doing in the readmes! Learn markdown.
Finally, Google scrum ceremonies and begin to follow them. Learn planning poker, rely on your team, and the three stand up questions. Do all this, and you will be ok.