My job & passion is to help mobile first-timers. Dealing with limited resources (or efficiently leveraging existing ones) is extremely common.

Building a working prototype to test the market is quite quick nowadays. The foundation of an app, especially before it is well-established should be clean UI and efficient task execution.

Considering (among other things):
- Apple likes when their native UI elements are used
- users like something they feel comfortable with upon first use
- there are many available apps you can use as a style/design reference
- you should avoid "stuffing" your app with additional features
- there is always room for UI/UX improvement but first, you need data/analytics to know what to tweak
- you mention "business plan", which makes me think the business itself might need some real validation

My advice is to hire a developer to release an MVP.

mobile app or a web app, the answer is both.
You and your service should (ideally) be available and reachable everywhere. A mobile application and a website optimized for mobile device are essential.

However, I feel like you're simply using the wrong terminology
I think you might be referring to native vs hybrid app.
In this case, others are right there are ton a of articles that discuss&compare both. Bottom line is current assets, time&money and functionality.

If you are asking, you should probably go with hybrid, which is what works for 99% of cases.
Hybrid offers not only easy cross-platform deployment but also much faster single-platform development and re-use of your existing developer team (or cheaper if outsourcing).
The most common reasons to lean towards hybrid is enhanced security or elaborate graphics (ie games).
Let me know if you want to chat more, I've already explained this a million times to clients.

Businesses (therefore also apps) are not sold, they're bought. It will be tricky to find a buyer while still having the upper hand. There are different things to trigger interest but it depends on your specific app.
The best way is to let them know about you and then start talking without writing you want to sell in the first cold email.

From my point of view you should usually stay away from comments that just state "the best is xxx". there dis no best, it all depends on your niche/target, user flow/experience and desired formats. There are a ton of ad networks, not to mention ad mediation and RTB to choose from.

Yes, Parse is undoubtedly the easiest, especially considering the vast array of functionalities and options offered.

As you already found out, there is no direct way of finding download numbers for a specific app.

Some companies disclose such information when milestones are reached or within interviews and blog posts. Did you try searching?.

If you're just interested in how competitive a category is, for ASO purposes, you can checkout app worth per category in SensorTower. I doubt the actual figures are accurate but it will at least give you an idea of how they stack against each other.

Alternatively, you can look for 3rd party articles like this: http://techcrunch.com/2013/06/27/how-do-you-break-into-iphone-app-store-top-50-try-23k-free-daily-downloads-950-paid-or-12k-in-daily-revenue/

Let me know if you want more help with you specific case.

Tom brought up some good points. I myself, when I was still freelancing had many clients who came to me precisely because of previous developer nightmares, especially hired through elance/odesk networks.

However, there are also other cases. Relationship/communication is very important, and so is price. Both might trigger wanting to switch developers.

To answer your question, you should make sure to have a list of external/3rd party frameworks used, how well-commented the code is and obviously access to the full source code.
Any respectable company will assess the source code before suggesting a re-code or estimating the time/cost load.

Feel free to reach out if you need help with your specific code.

depends who's building it but anything between $300 and $3000.
FYI the issue will be having it approved.

Nowadays it's very rare to get away with just one of them: you need both. If you're strapped for cash, consider PhoneGap which will allow you to re-use most source code across platforms.