You were claiming that deflation is a mirror image of inflation, right? Now you seem to be backpedaling this issue?
Not at all. Where do you get that ? Inflation increases nominal interest rate, deflation decreases nominal interest rate. Symmetric, no ?
We agreed to start at 0% inflation-deflation rate where nominal interests rates would be close to zero, then inflation-deflation kicks in.
Yes. If the nominal interest at 0% inflation/deflation is 0%, then the REAL interest rate is 0%. The nominal interest rate will then be equal to the inflation/deflation rate, because the real interest rate is unchanged, here 0%.
That means that if we have inflation of 5%, the (nominal) interest rate will be 5% ;
if we have deflation, the (nominal) interest rate will be -5% (which is, as I said, an absurdity, but which comes because of your starting assumptions of having a real interest rate of 0% and nevertheless deflation, which is economically difficult to consider).
In the no-inflation case, you buy for $1000.-, you pay 0 interest (nominal rate = real rate here is 0), and you sell for $1020 (if your ROI is 2%).
You make $20 profit.
In the inflation case, you buy for $1000,-, you pay $50 interest, you can sell for $1070, and you make again $20 profit.
In the funny deflation case, you buy for $1000, you pay minus $50 interest (that's what it means to have a negative interest) - that is, you only have to reimburse $950, you can sell for 970, and you AGAIN make $20 profit.
The only point being that nobody is going to borrow you money at a negative nominal interest rate. But that is because you wanted to consider the case of 0% real interest rate and nevertheless deflation.
If the business was profitable at 0% inflation, it will most certainly stay profitable in the inflationary environment as well, while in the deflationary environment it is not given.
I exactly calculated you the opposite.