Section 75 Claims only apply to agreements under CCA 1975. CCA regulated agreements are those which cover credit cards, including charge cards, HP, Finance, Loans, basically anything CCA regulated. Read your agreement it will state it if it is CCA regulated.
Now while debit cards issued in the 1990s were CCA regulated and unless you can prove it (personal experience) then they are not covered under CCA.
And to correct the person, the purchase of the item doesn’t have to be £100 on the card, you only need to spend £1 (or less) but something on a card and the entire item must be over £100 but less than £30,000.
This would allow you to enforce a Section 75 claim.
If you have two companies both covering then you can claim against both. For example, I purchased a car (not that I have). The deposit is paid for on the credit card, the rest of the balance is financed. Then I would be eligible to claim against both firms (as long as it was less £30,000).
Hence, for advise to anyone always pay for something using a Credit Card and pay it off if you are not sure. That covers you.
Full blurb http://www.moneysavingexpert.com/shopping/section75-protect-your-purchases for anyone that wants to find out.
As with regards to Monzo, the fact that they are using the MasterCard symbol that means they would be in some form of agreement under the license to use the symbol but abide by the rules. However, it is a pre-paid card thus this probably, and someone from Monzo would need to clarify this reduce their liability.
Pre-paid cards generally don’t fall under the protection of the FSCS scheme and thus you can’t complain.