Here are some answers to your questions that can hopefully get you on your way:
- Do applications have to undergo a reviewing process like Apple's appstore and the Ubuntu Software Center?
When you upload an app to the Ubuntu Store, it is automatically review by the store. Here, apps can take 5 minutes to go from review to being published! This is because apps are confined, so there is no need to have a code review. As a result, apps are published significantly quicker to the store than if they were published from Apple's appstore or the Ubuntu Software Center.
- Is it possible to monitor the activity of the app, in terms of number of downloads, users, etc.?
When you publish your app to the store, there is a page for developers to see how many people have download your app, on what date they downloaded your app, and from what region your users are from. Since you are making an HTML5 app, you could integrate analytic software, like Google Analytics to give you more information in real time.
- I don't have a smartphone, so will I be able to reliably test apps in the Ubuntu SDK?
The Ubuntu SDK has an Ubuntu Devices tab to manage Ubuntu Emulators, so you will be able to test out your application as if you had a real device. When you app is ready for testing, feel free to reach out to the Ubuntu App Developers Community on Google Plus to get people to test your app on real devices and get feedback.
Yes, the Ubuntu Store does allow developers to charge for their application.
- Can I get access to built in features of the phone/tablet such camera, GPS and accelerometer using HTML5? I know this is supported for android and iOS but not too sure about Ubuntu.
Looking on the HTML5 API Page on developer.ubuntu.com, it does not appear that there is a native api for access to these features. However, since Ubuntu Touch has native support for HTML5, it is very likely that some of these features are supported.
Your best place to start for access to these features is to check out the Cordova guide for Ubuntu. Cordova is an awesome api for developing cross-platform HTML5 apps with access to device sensors. For more information on Ubuntu HTML5 app development, see the HTML5 page on developer.ubuntu.com.
- Is there an official desktop appstore for Ubuntu Touch, or is the third-party uApp Explorer the only option?
There is an appstore, but it is in the Unity 8 version of Ubuntu, the version running on Ubuntu Phones. There is a desktop preview for Unity 8, but that requires installing it on your desktop. Eventually, Unity 8 will be default on the desktop. Until then, your will have to view apps from uApp Explorer or the store on the Emulator.
I hope this helped you! If you have more questions, see developer.ubuntu.com, the community, or askubuntu.com