If you're using this apartment as your office space, then yes, normally you can write it off as a business expense. You'll just need to prove it somehow. For example, declaring to the state that the apartment is for commercial use. I know that where I live, I also need to notify the landlord that I'm doing commercial work in the apartment. Double-check with state laws though! I am not a lawyer so I cannot give you any legal advice.

I've created a SaaS company and mobile app had the same issue as you. I'm telling you that it's not impossible. However, what I experienced is that some investors want an MVP and some don't. The most important thing is that you know there is a market out there that is willing to buy your product/service. So what we did was talk to as many people as possible about our project and created partnerships with large companies who also wanted to be integrated into our platform. There are websites that you can create an MVP yourself also which you don't need coding experience for such as Bubble.io. Websites like this can help you create at least a demo product that will allow you to show to your investors what your vision is. If you want to speak further, I'd be happy to setup a call with you.