In this poem of Emily Dickinson's:
1. How does the window open like a "pod"? What is a pod? And how does it open "mechanically"?
2. What is the "milliner" doing there?
3. "The man of the appalling trade", is he the one going to do the things needed before burial? I'm not sure of what they might be though in the Christian Religion. Why is he going to "measure the house"?
4. And, finally, what are "tassels" going to do in the "dark parade"? Are they those, as I remember, hung to the "coaches"?
These are off-the-cuff remarks, so take them with a grain of salt.
To me, the "pod" suggests that they open from the center lengthwise, like a pea pod. This suggests the opening of drapes, maybe shutters, and possibly of French windows. I get the feel of sudden activity after the death in which the house which had been shuttered and closed, suddenly bursts open with windows wide, rapidly, in familiar routine, automatically, "mechanically" performing the actions that follow a death.
The milliner and undertaker are measuring for their respective trades. The undertaker ("man of the appalling trade") needs to measure the doorways and halls to supply a coffin of the right size that will be able to be brought into and out of the house. The milliner will be making hats for the ladies to wear in mourning. Ladies wore hats in church, and still do in some circles, so they would need black hats for the funeral services and, indeed, many of them would be expected to wear mourning hats with veils for months. Hats of a certain era, Victorian among them, could be large, tall, and sometimes elaborate things. I think this is a bit of macabre and disapproving humor, mocking the practice of wearing elaborate hats for funerals, so elaborate that the milliner has to measure the house to ensure her hats will fit through the doorways, just as the undertaker has to measure for the coffin.
I think you are right about the tassels, they either refer to the coaches' dressings or the women's clothing, but I would check the scholarship and commentary on the poem before I could be certain.
I think there is a distinct feeling of contempt for those who dishonor the dead by making a formal show (hats, tassels, parade) or making money (minister, undertaker, milliner) from the death.