You cannot start the game as a Gaian or celestial. However, there may be the opportunity to play them later in game.
Medicine is dispersed under several skills. First aid generally comes under the survival skill, things like operating medical technology come under the technology skill. However, Omen is set on modern day earth - it is perfectly feasible for your characters to call an ambulance.
In terms of skills, you can counter stealth using your own knowledge of stealth. You may also use magic to counter a stealthy character (for example, if you have the aesthetics solution 'illuminate') but this generally has to be an active choice, rather than passive perception.
The meetings that form uptime are set in the UK. However, Omen is a game looking at world-changing issues and as such will have a fairly international scope.
Omen is set in an alternate universe where all genders are, and always have been, completely equal. Similarly, there is no discrimination based on sexual orientation or gender identity, and never has been. While there is a history of conflict based on ethnic divisions, there is not currently a problem with racism in the world. The idea of discriminating against someone based on any of this will seem absurd to a character based within this world.
Discrimination has shaped the world we live in today, and in order not to have to rewrite history completely we are saying it has historically existed. However, it does not make sense for your modern day characters to discriminate based on ethnicity.
The world can generally be assumed to be chugging along at a steady rate - specific political events are below the abstraction level of the game except where they directly intersect with character actions.
Your character is likely to be able to travel between countries with minimal difficulty, and will generally be able to get along reasonably well once they're there. Unless, say, they're a wanted criminal, in which case that's probably the least of your worries. English is a fairly common lingua franca, but by no means universally known.
Everything OC, along with character creation can be held to be accurate (barring GM errors). For everything else, what is stated on the pages is generally what is commonly believed by most witches at the game's start.