When decline is used as a transitive verb, it means "to refuse" or "to say no to": We are declining your loan application. - I regret that I must decline your invitation. Declining a customer would be a bad business move; declining your customer base is simply ungrammatical. Probably bad business too.
When a sentient actor (a person, a corporation, an intelligent animal) is the subject of decline in an apparently intransitive sense, there is generally an implied object; I would call this a "virtually transitive" use: I offered him a job, but he declined (the job). - We offered the chimp a banana, but she declined (the banana).
When a non-sentient noun is used as the subject of decline, it means that that thing/resource/quality is becoming less, or less powerful: The puma population has been declining for the past few years. - Hari Seldon says that the Empire is declining.
When a thing is declining, or a person's health or power is declining, we can say that that thing or person is in decline. As soon as his team started losing, he went into a decline. - This country's been in decline ever since they raised the drinking age.
When decrease is used as a transitive verb, it means "to reduce the amount of": I'll have to decrease my donut intake, or else my chair will break.
Sentient actors don't decrease intransitively; you can't say He decreased.
When a non-sentient noun is used as the subject of decrease, it means that that thing/resource/quality is becoming less: The puma population has been decreasing for the past few years. but NOT Hari Seldon says that the Empire is decreasing.
A crucial difference between decline and decrease in this last case is that decline can be used to indicate a loss of power, influence, significance, etc., whereas decrease can only be used for a reduction in quantity. Thus you can say both The population is decreasing and The population is declining, but while you can say The Empire is declining, you cannot say The Empire is decreasing, since there's still only one Empire.
Best Answer
JoseK is correct that the meaning of emend is confined to textual alterations, and that amend can be broadened to include the general improvement of other things. Still, amend is mostly used in the sense of improving text. If you're talking about fixing a rip in a shirt, you would be better to just use the word mend; to amend the shirt would sound strange. The Constitution of the U.S. may be amended (as it has been) but if the actual document itself were damaged and required restoration I don't think anyone would say the team that did the work amended the Constitution.