I'd say that, unless you're developing an entire history and language for your world, you may be over-thinking it.

Most place names have evolved over hundreds or thousands of years, and they changed as areas were conquered by invaders who spoke a different language. If you look at the history of the UK alone, there have been so many different peoples who've flavoured the names of the landscape (the Vikings, the Anglo-Saxons, the Romans etc, not to mention the local tribes like the Celts/Picts etc.) Wikipedia has some info ...

http://en.wikipedia.org/wiki/Toponym...om_and_Ireland
http://en.wikipedia.org/wiki/List_of...om_and_Ireland

To answer your questions specifically, I've given my thoughts below, though I'm no expert on this.

1. Could be either. Some leaders might name areas after themselves, but I think more often a geographical feature of the landscape would be the root of its name.
2. Don't think there's any specific convention to this. A river could pass through more than one lake on its way to the sea, and those would have different names.
3. See no. 1.
4. It would make sense for the biggest river in an area to have a name whose root word means 'Great River' or something like that, but again, it's hardly mandatory. The Thames river (allegedly) means 'dark' or 'cloudy' and takes its name from ancient European languages (depending on who you believe) but it has changed a lot through the years.
5. I think it's perfectly fine to have very different place names because of what I've written above. Names evolve through time, just as language does (compare Old English from a thousand years ago to modern English. They're barely the same language.) Some places will hold on to their original names, relatively unscathed, but others will be modernised, which could lead to big differences in style.

I found this site a while back, and found it really interesting...

http://www.ordnancesurvey.co.uk/reso...lacenames.html

It gives meanings for many different root words that crop up in British place names, and might provide some inspiration.

Tolkien is a bad example to follow for most of us, because the guy was a linguist, and words and their meaning were his joy, and he wanted to develop whole new languages which needed consistent rules and grammar. Most of us aren't doing that. If you decide 'Sathir' means big river, or, instead it's the name of a clan chief from 300 years ago, well, that's what Sathir is going to mean! It would be a nice touch if you could reuse root words in your map (for instance, decide that syllable 'xyz' means 'hill' and use that element as part of the name for a few hills throughout your map. It gives a sense of consistency and creates the idea of a different language used by the natives.

So, in summary, I'm no expert so take these thoughts or leave 'em, but really, you can call your places/landscape features whatever you want, depending on what suits your world-building. If you want to name lots of places after a dominant figure from history, go for it. If you want to draw attention to an ancient battle, name something after it. If these historical elements are important to the story, then work them in through the place names. However, if there aren't any memorable leaders/battles, just make something up. It's entirely up to you.