Well, you know I'm aboard...
"If I split my Space Marine Tactical Squad into Combat Squads, can I then mount both units inside a Rhino? What if I have a Razorback? What if either unit starts in Reserve?"
No, once split they are considered separate units, and the Transport rules clearly state "A transport may carry a single infantry unit". If you have purchased a Razorback, either Combat Squad may begin the game deployed inside, but you may not do so if not split into Combat Squads. A unit in Reserve can be mounted inside a transport, but this must be declared at deployment. The same rules apply as outlined above.
"If an Independant Character, such as Eldrad Ulthran, joins a unit with a Dedicated Transport, do I have to declare to my opponent which vehicle is the one he is inside? If I have several identical vehicles with identical contents, excepting the Character, it becomes impossible for my opponent to tell from my army list, so should I tell them before the game?"
The Rulebook states "always make clear to your opponent which squads are embarked in which transport" The key word here is always - you should indeed inform your opponent which squad he has joined, and where the squad is - by extension, you divulge his location.
One more before I go to work:
"So here is the scenario Player A has Khan which allows all of his units to outflank. Player B manages to line the whole edge of the table on one side with his units. There was not a single inch that was not covered by Player Bs army so when Player A tried to outflank and happend to roll a 1 (where all of Player Bs units were) he couldnt get in. What happens to the guys that were trying to outflank? Do they just die do they not come in do they auto go to the other side?"
The units would be destroyed, unless they had a method of moving over your models (such as Jump Infantry) or could move yours aside (eg Tank Shock) However, you can expect to be banned for poor sportsmanship if attempting this manoeuvre, and to have the victory handed to your opponent instead - is it really worth it?