Search for a WYSIWYG, non-latex Online collaborative paper writing Tool
Considering all of your requirements, I would suggest the following solutions, all of which essentially are either based, or focused on (compatible with) using Microsoft Office 365 software:


Microsoft Office Equation Editor (see this, this and this);
Microsoft Mathematics Add-in (in the context of this question, Microsoft Mathematics software provides an extended collection of symbols and structures for mathematical expressions; see this page for more details);
MathType by Design Science (this is the most comprehensive solution, which is compatible with Office 365 environment, among many others; in fact, Microsoft Office Equation Editor is a simplified version of MathType).