Sami, in spite of my doubts on the option of considering slots as an asset, I think you have done a great job on the balance sheet: congrats!
However, is there still room for discussion on the cashflow statement?
Cashflow from financing activities is related with the B/S financing structure of the airline (loans withdrawals and repayments), whereas cashflow from investments is related with transactions on B/S fixed assets (aircraft purchase/sale, + slot fees as per your accounting option).
I do agree with other posts suggesting that aircraft leasing income should not be classified as investing activity, and in particular a "capital gain", but rather as an operating cash in the same way leasing expenses are.
By the way, in the manual, the definition of cashflow from investing activities reads: "Net investing cash flow is designed to show uses of cash related to transactions that impact the capital stock of an airline". It shouldn't read "Fixed Assets" instead of capital stock? What do you mean by "capital stock"? This term is commonly used for the issuing of shares.