Are there legal implications of using these securities exchanges as we are essentially a private company (as are most who list there AFAIK).
There are very serious implications. Consider that these are illegal in many countries. They have not been up long, and exchanges of these types tend to come and go. If your business is already profitable, but at a small scale, then it's likely your business will outlive any exchange you list upon. Listing here will hurt your prospects with traditional VC. If you still want to list on one of these fly-by-night exchanges, then at least spend a small consultation fee ahead of time on a lawyer so that you know what kind of liabilities you're opening for yourselves.
Traditional VC are more interested in growth in value and a potential exit further down the line (during later series funding or an actual, legitimate IPO) once you make yourself attractive to a buyout. They fund many projects with the expectation that many will fail, but that the ones which do survive will more than make up for the losses. They will also need a bit more wooing than making a slide deck and a forum post. Considering that they are investing in the idea and your ability to implement it, they will need to have some evidence that both of these are sound. A profitable product at minor scale is a good start, but you'll need to prove that you can scale and that the market for the idea exists or can be created.
Most incubators offer more than just startup capital. They'll offer technical, financial, and legal expertise (usually in the form of consultations or mentorships), contacts, and will possibly help you seek later stage funding when the time is right. They, and the VC they represent, will want to have some level of involvement at every stage until their exit.
When you list on one of these unregulated bitcoin exchanges, though, you will find different types of investors.. Many are used to the constant swings of BTC price and are not looking for a long term investment. They will buy into these pseudo 'IPOs' with the hope that it's been priced incorrectly and that demand will turn a quick profit once all of the available shares are sold. IPO pricing is a skill. If the market for an IPO is incorrectly gauged, then you will either not raise enough capital or will be leaving money on the table. Others are more traditional, in the sense that they'll invest without sure profit being generated and wait a while before expecting any dividends (see Neo&Bee lead up, though this was a bad idea to begin with), but these will be hoping that their share price is going to rise commensurate with their wait. Some are just hoping for a solid source of dividends and will focus on those businesses already generating profits while maintaining their share price. With any of these, though, be prepared to answer to the criticism of anonymous investors (or bystanders).
I can't help you on this one. ROI will differ depending on the share price that an investor bought in at. If you are looking to provide a flat percentage return, consider taking out a loan instead of selling equity.
You need to do a lot of calculation. Take a look here for some common metrics that you should be reviewing to help you determine the cost of a customer through various channels, their lifetime value, churn rates, etc, all so that you can predict revennue and costs. Beyond that, you'll need to somehow figure out how bitcoin price volatility impacts your business (or if it does at all).
Investors will need to see some evidence that you can follow through on your plans. This can be evidenced by a history of experience, previous successes/failures. They'll want to know that the plan itself is sound. Investors will want to know that you're accountable in some manner; in the bitcoin space, it's too easy for someone to take investment and then slip away.