Identify food items with a decrease in both month-on-month and year-on-year percentage change.

We look for negative values in both "YEAR ON YEAR" and "MONTH ON MONTH" columns.

• For 750 ml sunflower oil: Year-on-year change is $-12.7\%$ and month-on-month change is $-2.3\%$. Both are decreases.
• For Oranges per kg: Year-on-year change is $-8.9\%$ and month-on-month change is $-14.5\%$. Both are decreases.

The food items are 750 ml sunflower oil and Oranges per kg.

Value A is the total price in May 2022. We sum all the prices in the "MAY 2022" column.

$$A = 12.60 + 45.56 + 52.97 + 40.68 + 22.07 + 37.73 + 86.80$$ $$A = \text{R}298.41$$

The missing value is $\boxed{\text{R}298.41}$.

From the table, the price of "Eggs 1,5 dozen tray" in May 2022 is R52.97.
Since $1.5 \text{ dozen} = 1.5 \times 12 = 18 \text{ eggs}$, the price for 18 eggs is R52.97.
To find the price of 1 dozen (12 eggs), we divide the price of 1.5 dozen by 1.5.

$$\text{Price per dozen} = \frac{\text{R}52.97}{1.5}$$ $$\text{Price per dozen} = \text{R}35.3133...$$

Rounding to two decimal places for currency:

$$\text{Price per dozen} = \text{R}35.31$$

The price of a dozen eggs during May 2022 is $\boxed{\text{R}35.31}$.

Price of oranges in May 2022 = R22.07
Price of oranges in May 2023 = R20.10
The ratio is May 2022 price : May 2023 price.

$$\text{Ratio} = 22.07 : 20.10$$

To express this in the form $1 : \dots$, we divide both sides by the first term (22.07).

$$1 : \frac{20.10}{22.07}$$ $$1 : 0.91073855...$$

Rounding to three decimal places:

$$1 : 0.911$$

The unit ratio is $\boxed{1 : 0.911}$.