But can they really refuse to load my baggage because of that?
That's irrelevant. They have in fact done this. You'll have to deal with it.
How did my bag even make it to Beijing then? It passed through customs at A, and made its way to Beijing without any problems.
Your bag did not pass through Customs in A. It passed whatever security screening exists ...
You will indeed need to collect your bags yourself and transfer. Most airlines do not have a method of transferring between different airports in a city.
Indeed, Asiana has this on their website.
They have a service if transferring to a domestic flight, even if at Gimpo, but (possibly due to regulations for international carriage), do not offer if ...
If the plane is on time, this is doable with Global Entry, hopeless without. Upon landing, you might see a queue a mile long waiting for immigration and empty space next to it. This might be signposted for crew only or not at all but don't be disheartened, by the time you actually get to immigration (this can be a few minutes walk next to all the people who ...
You should be alright, your connection time is generous. But if your incoming flight has serious delays you might run into some trouble. In that case you may need to go for the fast option rather than the cheap one, or be late for your second flight and miss it.
The trains, both subway and LIRR will be busy and can get crowded at peak times, but the locals ...
(1) Do I require the transit visa and from where I would get it (Gimpo Airport or Korean Embassy in Osaka, Japan)?
Depending on your citizenship, if you are not visa free to Korea you will need a transit visa as your layover is longer than 6 hours and you will be switching airports.
You would apply at the Korean embassy in Japan.
(2) How would I ...
In answer to the second of your questions, there is a train between the two airports that takes approximately 35-40 minutes. It costs approximately 3500 Won.
I've had a look regarding your first question and I can find no evidence that visas are available on arrival in Gimpo. As an Indian national you would require a visa to enter Korea (which you must do ...
It's definitely worth checking out how far your hotel is from the Metro. You pay a bit of an airport premium taking it from BCN. For a lot of people, it's still going to be the sweet spot between cost and convenience, unless you have an excessive amount of baggage.
Given the current political situation (as of the 15th of October), I'd watch carefully the news to see how events unfold. Do not assume infrastructure will work according to plan.
There are plenty of political rallies against Catalan separatist leaders' prison terms. Some of these demos turn violent, and they are blocking partially the roads to the airport ...
Most efficient way
The most efficient way is to take a taxi which costs around £25 one way. (Around £6.25 per passenger). The taxi takes 14 minutes.
Alternatively, if you want a cheaper way, you can take the bus A2/46 which takes around 30 minutes and can cost as little as £2.
Alternative routes can be viewed on Rome2rio.
I went to Montevideo a couple of months ago and found the buses from the airport quite pleasing and not at all as you have heard described - the bus I used to get downtown was not full at all, there were in fact lots of empty seats. I arrived in the afternoon.
It took about an hour to get close to my stop, which is past Tres Cruces - but you said you didn't ...
As per a comment above, Rome2Rio offers several options:
Bus - bag may be a problem, but cheapest of the options
Taxi- Would work, but more expensive
Drive - not really an option unless you can hitch
Uber - middle of the range, and available.
It's up to you as to which of those is more suitable to your purpose/budget.