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.
2 - The blockchain only shows how much Ether you have received from each investor. No tokens exist yet, so the balance on your website is purely cosmetic.
4 - How can you possible believe that your manual approach is more secure than an automated contract? You are basically denying the underlying principle behind the Ethereum network.