Whew, you're reading Dickens. A lot of native English speakers can't handle Dickens, so congratulations to you already.
Let me go in order.
We had everything before us...
To "have something before you" means to "have something in front of you," that is to say, the future. Another way to say this is, "We had everything to look forward to, we had nothing to look forward to."
We were going direct to Heaven...
"Heaven" should be clear here, but because heaven is "up", "going direct the other way" is a delicate Victorian way of saying "hell," which is "down." So, "We were all going to heaven, we were all going to hell."
its being received...
Not a simple construction. "To be received" means "to be considered," "to be thought of," etc.
"The superlative degree of comparison" is something you've probably already encountered in learning English. "Best" is the superlative of "good," "highest" is the superlative of "high," etc.
So Dickens is saying that the past was so very different than his own day that the "authorities" of his time, probably historians, would only think of it in extremes, such as "best" or "worst" - with Britain on the "best" side.
To say it another way: ...in short, the period was so different than now that our historians insisted on considering anything good of that time as the absolute best, and anything bad of that time as the absolute worst.
In other words, Dickens uses the end of this sentence to explain the pairs of extremes he began it with (best/worst, wisdom/foolishness, etc.). He's saying, "These are the historians' words, not mine."
Does that help?
Understandable = behaviour or reactions which seem normal and reasonable, as in
His unwillingness to go through all that again is quite understandable.
It is understandable that parents are angry, and looking for someone to
blame.
Replace it with comprehensible and the simple sentences take on an odd, stilted air.
On the other hand, comprehensible rather implies intelligibility than normality (the more so, since the word comprehensible is heavily used in scientific papers):
The epistemological project feels like the pursuit of a perfectly
comprehensible intellectual goal. The explanation of the science at
work was clear, concise and comprehensible.
Replace it with understandable and the latter begins to seem a bit out of discourse.
Best Answer
A reason is a thought that is why someone does something. A cause is something that makes something else happen. Reason should explain something, cause might not, it might just be due to something else without an explanation.
They're usually treated the same, though.