What logical reason would they have to completely ignore an entire section of the world?
ITTL, Britain, France, USA and Russia are all too busy fighting each other (there's four wars of Napoleonic-WWI scale in the nineteenth century), with a total war mentality that means merchant fleets and stuff get wiped out every 20 years. Peacetime in between is mostly used to prepare for the next round, which both sides accept is inevitable. So to begin with, they probably can't afford a large-scale effort into the Far East.
In such a political atmosphere, neither side really wants to provoke the other side into starting a war (which they very well might do). Declaring a huge chunk of the world as part of your sphere of influence is a fairly large provocation that is not going to be received well by the five million men with guns on the other side of the Channel.
Also during the 1810s-1830s, the attitude of both OTL China and Japan was pretty strong isolationism. If Europe isn't pushing to open them up, neither Asian power has any real reason to change that: they're doing fine as they are. ITTL these attitudes become more radical and more determined as time goes on. That means that if Europe wants to interfere, they will have a fight on their hands. Which they probably can't win without taking enough forces away from Europe as to expose their homeland to invasion. China had a fairly large army at this time, and it is unlikely that Japan will be "opened" without China first either giving approval or being beaten down.
Of course, as soon as France loses its powerful position in the world (1887 ITTL), Britain and Russia go full bore on colonising. Africa first because it's closer and the natives are easier to fight, before China/Japan which are only considered once infrastructure has been set up in the Russian Far East and the Philippines (owned by the UK).
There's no logical reason to ignore a potential market, possible ally or somewhere to exploit.
Yes there is: if your neighbour says "don't go there or I'll smash your head in" and is strong enough to make good on that threat.
Politely, that's absolute nonsense.
So I have to disagree with you there. In a close-to-OTL scenario, your points are very reasonable. In the Napolead, with its very different climate and attitudes, other ideas will work better (or at least are perceived to).
- BNC