If by "suspicious" you mean they're trying to get free work or free ideas from applicants and if they get hired, they will be exploited, I would answer NO.
If by suspicious you mean you will probably be asked to work more than 40hrs a week, and sometimes on week-ends or nights, I would answer YES. Because this is a startup, and because they seem to offer twice the average salary, it feels somehow logic to me.
Now what you should do really depends on the kind of job you're looking at...
the long version now :-)
I agree it doesn't sound very exciting as such. Their recruiting process is clearly not too good, which already can give an indication on the quality of your future colleagues.
- On the pushy questions:
I think although it might not be a good thing to ask for so much details in the interview, there's probably no hidden agenda behind it. If they wanted to do some kind of brainstorming to get ideas on their next step, they might as well ask on the web and get a lot more of answers of very various kinds, so they're probably not "tricking" applicants
- On the 8h coding test
Although I find it quite long, I don't consider it too uncommon neither. From you're description I'm not quite sure such a task can be accomplished in 8hrs, so this might be a way to see what kind of quality/quantity you can deliver in 8hrs. I don't think this is for you to do free work, because as I said I don't think you can get a quality result in just 8hrs, and even if you would, there's probably not a lot of money to get out of it should they sell it to clients.
- On the "working on week-ends and extra hours"
Well at least they announce it clearly. If that is going to be an issue for you, maybe the startup world isn't the right one for you, as it is quite unusual for startup employees to do a 9 to 5 with no extra-hours ever. Some people have no issue with that, some would want it compensated somehow, some might just refuse it, it's all up to you to decide which one you want to be.
Note also that even not compensated directly doesn't mean you're never gonna get any reward: in a fair-play management style (and I've been under such management sometimes) your commitment could be rewarded by being proposed a raise, some shares of the startup, a higher-level position, etc... And in this case you mention a salary being the double of the average salary for such a position, this is for me an indication that they expect you to work more than in another similar job.
It doesn't mean neither that you would be "blamed" for doing a 9 to 5, but it certainly plays a role in how you would fit with your colleagues. Coding nights or week-ends, as strange it may seem, are sometimes helping the team spirit: you've gone through some harsh times together, but you deliver something on time, that accomplishment as a team is sometimes very rewarding in itself.
The many things you may learn on various fields by working in a startup may also be considered an indirect benefit...
So, does it look like a scam ? Not to me. Does it look like a company that will be exploiting you ? Neither does it to me. Will yo be a good fit, and will that company be a good fit for you? Only you can have an idea about that, and even then you might even realize on the job it's much better or worse you thought in the beginning. I've been dreaming during my studies of a job were I would be paid and not too stressed. I've had such a job after my studies, and quickly realized it was not good for me and it's not what I want at all. Knowing what will make them happy is one of the hardest things for human beings...