How would debt work in a bitcoin only economy, and why would anyone ever spend?
Question: how is this planned to function in a bitcoin only future???


They can use their assets as collateral for the loan. 

The price of Bitcoin (measured in chicken eggs, for example) doesn't necessarily need to increase when somebody takes out a new loan. The price of fiat doesn't change when people take new loans either. 

(If workers become more productive and their pay correspondingly increases, the employer may require more money to fund their working capital, but their cost structure doesn't change - the workers help them earn more and the employer pays them approximately the same percentage of their earnings as before. There it is unlikely that everyone would have to use Bitcoin - if Bitcoin gets used on a large scale, that means multiple crypto-currencies would be in circulation.)


  Second Question - if what I suppose in the first question is in fact the vision (please correct me if im wrong!), why would anyone ever spend?


Gold mining generated approximately the same amount of new coins as Bitcoin mining and people would still spend their gold (and silver and stock certificates and diamonds and other savings and assets).

Things worked well before today's fake fiat money existed, and will work equally well after it gets destroyed. 

You can learn about it from books and materials discussing how it was before central banks instituted state-sponsored plunder.