Who is the seller in a share buy back program?

This is a bit an academic question and will not help with your investment but it could help to understand how the financial system is working.
In Switzerland when there is a share buy back by a company with the objective to destroy the shares and to reduce the number of shares then the difference between the nominal value and the transaction value is taxed by the 35% withholding tax. As Swiss tax payer you can recover this 35% as long as you declare this transaction as income on your tax declaration. If you live outside of Switzerland then the withholding tax can be refunded as long as your country has a treaty of double taxation with Switzerland but the process may well be a burden.
My question is then : Who is selling the shares on the second channel opened for share buy back? Pension fund, charitable organisations?

The owner of the shares sells them to the bank charged by the company for this. The second channel allows to identify the seller for tax purposes. Otherwise the company would have to pay.

The shares may be sold up to 5% higher on the second channel with respect to the market.

As such a buy-back is a partial liquidation of the company for tax purposes, the difference between the buy-back price and the Nennwert (issue value) is taxed as income. Therefore, private investors are disadvantaged by a second channel, as professional investors are taxed on their capital gains anyway.

If you are a really good customer, your bank might buy the shares from you for mutually advantagous conditions and sell them on the second channel itself.

So you basically gave the answer yourself: professional investors or maybe non-taxed entities.