The project won't run on the Ethereum network.

1 - That makes no sense. You can still use a smart contract for the Ether payments and your own system for the BTC payments. The benefit for your investors is that with a smart contract your tokens will be automatically and certainly distributed, while with your approach there is only a promise and nothing else. The whole point of using a smart contract is to avoid the middle man, which is you. You offer a project based on the Ethereum network, yet you don't use the one benefit it provides. That is counter-intuitive and can be interpreted as a possibility to scam.