Calculate simple interest for 1 year and total amount after 2 years, and compound interest for 2 years.

Principal ($P$) = R15 000
Annual simple interest rate ($r$) = 6% = 0.06
Time ($t$) = 1 year

The interest that will be added each year is $\boxed{\text{R}900}$.

Principal ($P$) = R15 000
Annual simple interest rate ($r$) = 6% = 0.06
Time ($t$) = 2 years

The amount available in his account after 2 years will be $\boxed{\text{R}16800}$.

Principal ($P$) = R15 000
Annual compound interest rate ($i$) = 5% = 0.05
Number of compounding periods ($n$) = 2 years
Use the compound interest formula $A = P(1 + i)^n$.

Amount with simple interest (6%) = R16 800
Amount with compound interest (5%) = R16 537.50

Since R16 800 > R16 537.50, Thabo would have less money with the 5% compound interest deal.
Therefore, the manager's claim is refuted. Thabo's current deal of 6% simple interest is better.

Principal ($P$) = R12 000
Annual simple interest rate ($r$) = 5% = 0.05
Time ($t$) = 3 years

If simple interest is added, Pinkie will have $\boxed{\text{R}13800}$ in her account.

Principal ($P$) = R12 000
Annual compound interest rate ($i$) = 5% = 0.05
Number of compounding periods ($n$) = 3 years (compounded annually)

If interest is compounded annually, Pinkie will have $\boxed{\text{R}13891.50}$ in her account.