Yes, citizens of Cameroon generally need a C-Visa to enter the Schengen Area for a short term visit.
Your future wife is a family member (Mother in law) of a EU Citizen (polish) residing in the United Kingdom (which is not a Schengen Member state) .
- has an Article 10 or 20 Residence Card as a dependent family member
- the EU Citizen, against which the residence card was issued, will not be traveling with her
When a family member is not travelling with or joining the EU Citizen, a C-Visa (Schengen Visa) is required. Such visas are generally issued under special conditions
- free of charge (3.4.2 Visa Code Handbook)
- an appointment is often not required and issued swiftly
- multiple entries and valid for a longer period
Such an application must be made at the Consulate of your primary destination for the first visit (i.e. where you will be staying the longest or, in tie situations, the first).
You should check if your future spouse allready has such a visa and that it has not expired.
With such a visa, the 90/180 rule applies.
In theory, once you have married, she will become a spouse of an UK Citizen who, until the 31st of December 2020, will be treated as a EU Citizen within the Schengen Area. When traveling together your spouse would not need a visa during 2020.
So taking a verified copy of your marriage certificate with you should cover you both ways, insuring that any pedantic technocrat will find no fault, since a new United Kingdom residence permit will unlikely be issued in time for your honeymoon.
One would hope that a certain grace time exists for such situations, but I know of none.