Who would buy bonds and with what if in this scenario we all didn’t believe in the green back?
The question I expected you to ask is why people wanted the bonds, since all they're backed by is future dollars. The circularity is resolved by the fact that people know that there will be some demand from the government for dollars, since the government has outstanding dollar-denominated debt.
I think an important cause of demand you're missing is that people still need to pay taxes in dollars.
> the government would still need to exchange those bitcoins for dollars in order to pay its debts
That is not true. The government does not collect taxes to pay its debt, as it can simply spend its own currency which it creates. Taxes are relevant to spending in that it keeps dollars valuable.
Not if it wants to hit an inflation target.