It sounds like you need to sit down and figure out some strategies for your goals. It is great that you want to get into investing, but how are you going to do that and what is the best avenue to get there? Just jumping on an offer because you are ready to move doesn't sound like you have thought this through very well.
I am going to estimate that your mortgage payment is in the $800 range, plus the association dues would put your total payment at $1136 (ish). If you rent for $1500/mo that would net you $364/mo. Keeping the condo would have given you 2 doors of rentals vs just one if you sold and bought a duplex. You are in a position to be able to own two properties if you play your cards right. The conventional loan option you mentioned above allows you to close on a second property if you have a signed lease on the condo. It is 100% possible to have a tenant lined up to move in as soon as you move out.
I would talk to your realtor about what would happen if you chose not to sell the condo. There are consequence to breaking a real estate contract, and your areas specific PSA (purchase and sale agreement) should spell that out. Generally you can't just break the contract. You could be sued for non-performance. This might be an instance where you have to chalk it up to a learning experience.