Create excel cell short cut calcs for users

I've been building a spreadsheet to calculate financial instruments (fill a table with notional, rates, dates etc.) the bulk of the spreadsheet I have built, however, I've got to a point where I need to have custom functions to make the sheet easier to use.

For example rather than typing "5000000" or "5e6", I want to be able to enter "5m" for 5 million. "b" for billions etc. This expect to be very straight forward.

For dates I want the user to be able to enter "5y" or "5m" or "10d" in a cell and for it to be able calculate the "5y" date from a "spot" date (defined by a currency and holiday, usually 2 business days ahead). Also for dates (the tricky bit) I want to be able to enter a code for IMM dates. For example I want to be able to type IMM2 (for the calculation to look at the currency in another cell, and return the 2nd IMM date it being 3rd Wednesday in march 2019). I also want in the same cell to be able to type "sep 20 imm" for it to return the 3rd Wednesday date in sep 2020.

I also looking for a solution to a circular reference. I have two cells side by side. One is the input for the other. I want cell A to drive the value of cell B, but also want to be able to over type cell be to output cell A.

I have a formula in a cell. If i over type a number over that cell calculation are driven from the over type. Now if i want to revert back to the formula I want to type "a" (or another text entry).

Happy to discuss to help you understand the project.