First Timer - App Advice Needed

PREMISE
fyi, some minor details are left out to protect the premise. but in general, the purpose of the app is to create a countdown clock. and we intend to have different sponsors.

FRONT-END REQUIREMENTS
when the clock expires, it will have beep and vibrate.

to make money, when someone brings up the app on their phone, they would see a coupon from a sponsor company. and upon the launch screen, if the person likes the coupon, they can request to have it e-mailed to them. there's two options to have at this point. a) the app sends me (the app business owner) an e-mail confirming that this individual has requested this coupon. and I'll e-mail it to them whenever I get a chance [but this could get messy if there is a ton of users] b) the app somehow automatically send an e-mail right then and there to the individual [this could be a Day 2 project].

regardless of whether or not the visitor has requested the coupon e-mailed to them, when they click the "ENTER APP" button, they should be transferred to the APP SCREEN.

BACK-END REQUIREMENTS
a. we will NOT store any customer data or e-mails or anything else. we will simply generate a one-time e-mail to that address and that's it.
b. we would like to track a) how many downloads so we can tell potential sponsors b) how often people use the app and how long it's open on their phone for.
c. needs to work on android, IOS and windows phones. we do not care about tablets or computers. smart phones users will be 99% of our audience.
d. we need some sort of back-end or web interface where I can enter in new sponsor names, logos and coupons.

QUESTIONS
1. I'm not going to learn to build this myself. i have no knowledge of how to build one and don't have the time to learn. i plan to hire a freelancer through odesk.com. what program should I ask they use to build this? I've heard terms like swift, xcode, ruby on rails, twitter bootstrap, etc… ideally, is there one go-to popular program that creates a cross-platform compatible app? my fear is that if I have to drop a programmer in the middle of the project and pick up another, will the new person be able to pickup where the last guy left off?

1. how would I go about changing the coupons? and changing the sponsor banner ads? would there need to be some sort of web interface? would I need to purchase a website and hosting account and have some functionality built there? is there some dashboard somewhere else?

2. how big scale a project is this? roughly how many hours should this take a COMPETENT app developer? seems to me like one of the most basic apps you could build but what do I know.

3. at what point does an app get submitted to google play? apple store? windows whatever they have? or do I even need those entities or could I just let people somehow download it from a website? if so, what would I be missing out by not getting it listed under those marketplaces?

4. anything else I have not thought about that I should be aware of?

please advise. thanks in advance!!!