Full check-up on a struggling houseplant

An indoor plant has been slowly declining and you cannot pin down which thing is wrong.

Give my struggling houseplant a full assessment, working through the indoor variables systematically.

Go through each and tell me whether my described setup is right, borderline, or wrong for this species:
1. Light, in real terms. How far from which window, facing which way, and what that actually delivers.
2. Water: frequency, volume, drainage, water quality, and whether the pot can actually drain.
3. Humidity and temperature, including radiators, draughts, and cold glass in winter.
4. Pot and compost: size, material, age, and compaction.
5. Feeding: whether it is fed, with what, how often, and the season.
6. Pests, including the ones that hide underneath and in the compost.

Then rank the problems by how much they are contributing, name the one change to make first, and say what improvement I should see and when. If it will not recover, say so and tell me whether anything can be propagated from it.

Plant:
My setup, answering as much of the above as I can:
How it has declined and over what period:

Diagnosis and rescue. Working out what is wrong with a plant, what is eating it, and whether it is worth saving at all.