1. "Merchant Account" is defined the same as for a bricks and mortar business--that is, a Merchant Account is simply a business's ability to accept and process credit cards.
Not really, a merchant account is just an account. The ability to process the payments requires technology.
Online you need to be able to process payments and you can do this with or without a merchant account and the main thing you need is the processing service itself.
2. "Yahoo! Stores" is on the same plane as Miva. That is, both are services which help someone set up an online store.
Miva offers subsets of the total package as well. But as far as I know, yes, both offer total packages with hosting included.
3. the "Services" you speak of are more-or-less "kits" to set up and run an online store, and are generally inclusive of the software needed to build the structure of an online catalog, shopping cart, and online credit card acceptance.
Think of them as remotely hosted solutions.
If you do it yourself you need to be able to put together and keep together the applications, if you go with a remotely hosted application it's streamlined but you may pay more in the long run and you have less autonomy.