Urgent question on convertible debt

Basically I find in question 2 that a debt investor gets a 3,937,500/20,000,000 = 19.69% ROI (assuming no discount).

Now question 3 basically refers to convertible debt, but I can't find a common conversion premium (debt --> equity) that would provide to ALL investors a ROI of at least 19.69%. It depends on 1) the principal investment 2) the amount of interest already received.

In such context, the conversion premium that I receive if I decide to convert the debt after 4 quarters will obviously be different from the one I receive if I decide to convert it after 6 quarters.

Am I overthinking it? Underthinking it? Would greatly appreciate any help, thank you in advance.