Learn English – “Bitter line” Meaning

"coiner" - one who counterfeits coins. Not something we hear of much today, but in earlier times was a reasonably familiar sort of criminal.

"box-room" - a room in a house (usually a small room) which is just used for storing things, usually things that are not wanted very often. The collocation "box-room attic" is unusual, because these two words mean nearly the same thing. (Not quite, because a box-room could be anywhere in a house, not just under the roof).

"cistern": yes, it is still quite common for British houses to have a water-tank in the roof space - sometimes two (hot and cold). This was partly to provide a head of pressure for taps and showers, and partly to guard against interruptions in supply (and in the case of hot tanks, to provide a reservoir of heated water for when you needed it quickly, as in a bath).

There is not a literal tunnel, it is saying that between the tank and the sloping roof there is a long and narrow dark place (dark because it is behind the cistern).

"of course" is a parenthetical remark meaning "as you already know", or "as is obvious". It is much more common in speech than in writing, but of course this writing is meant to suggest somebody is speaking, and telling a story.

"For" says that the sentence is a reason or explanation of what precedes. So the meaning of "For of course he was thinking .. " is something like "(He was excited) because, as you probably realise, he was thinking ... "

In short, the narrator is saying

You (the little deer) have just died and gone to paradise. I know you are playing there and ready to run off to your eternal enjoyment -- but hold on a moment -- don't run too far ahead of me: just let me order your grave to honor you, then I'll die as well and join you in the afterlife.

This is possibly confusing for a couple of reasons (outside normal poetic license and archaic language). In particular, "to bespeak thy grave" is to "order a grave for you"; that is "purchase your plot and commission your tombstone or monument".

The following stanza makes this a bit clearer:

First my unhappy statue shall

Be cut in marble, and withal

Let it be weeping too; but there

Th’ engraver sure his art may spare,

For I so truly thee bemoan

That I shall weep though I be stone;

Until my tears, still dropping, wear

My breast, themselves engraving there.

There at my feet shalt thou be laid,

Of purest alabaster made;

For I would have thine image be

White as I can, though not as thee.

That is, "I'm going to order a monument to be placed over your grave; it will be a statue of me, weeping, and standing over your body, carved in alabaster, dead at my feet; but neither my image's tears nor your image's whiteness will do justice to the real thing (my sadness or your purity)."