I have questions about the usage of でもある and さっそく in this context

Question

Here's the sentence:

魔法のメッカでもある魔界に、（観光で）いくことに決め、さっそく靈夢にないしょで出かけていったのであった。

I understand what's being said: "Since, Makai was the Mecca of Magic, as a sightseer, [she] decided to go [there], but excluded [telling] Reimu Hakurei."

I'm just confused about the function of certain phrases in the sentence like and the placement of certain words:

1. でもある

Is it used because other things may or not have been said previously about the subject in focus, 魔界?

2. さっそく

I get that the phrase is supposed to function in this sentence to the effect of meaning 'rushedly', or excluding, or 'cutting steps out', but the action of 'not telling' the subject in focus (靈夢) is never explicitly mentioned. Is the action of telling her being purposely omitted in this sentence?

3. いったのであった

Is this all one expression? Why is ので placed so far-back in the sentence? My understanding of ので is that it's supposed to tail the part of the sentence that's supposed to be the reason, not the consequence. Why then is it placed all the way at the end of the sentence? Is going to 魔界 without telling 靈夢 not a consequence of 魔界 being the Mecca of magic? Is 魔界 being the Mecca of magic, not the reason she is going there?

Also あった does it roughly translate to (as it happens) in this context? I'm confused.

Can someone please explain this all to me??

Answer 1

1. That も means "also". "Makai is also (known as) the mecca for magics ..." I don't know how 魔界 looks like in this franchise, but it's usually not the kind of place where people willingly gather and learn magic. So I think "also" is used to indicate 魔界 is not only the chaotic place where monsters live.
2. さっそく is "without delay/hesitation" or "at once", and it modifies 出かけて行った. Perhaps "rushedly" is too strong as a translation.
3. 行ったのであった is the past tense of 行ったのである, which is a more assertive version of 行ったのだ, which in turn is a more emphatic and explanatory version of 行った. This ので is the te-form of so-called explanatory のだ (see this answer for more info).