How you understand them includes a couple of misconceptions, so please forget about it for a while.
ていく and てくる have three usages (each of which is more frequent in this order).
- to do something and go somewhere/ come back
- to do something to an outward / inward direction
- something changes, repeats or accumulates gradually or step by step
As you see there's no "/" in the third usage, both ていく and てくる refer to the exact same objective situation then. For example, 空が明るくなっていく and てくる mean "the sky will get brighter and brighter". However, you can also say them when the phenomenon is happening, where most English speakers will say "the sky is getting brighter", which focuses on a slightly different point, though.
The difference is if the speaker is interested in the result. When you say 貧しくなっていった, it doesn't really tell how it went after, besides the person got poorer and poorer. When you say 貧しくなってきた, however, it reflects the speaker's sense of vulnerability and usually means the influence of the result remains as it is.
Their past form means either that the gradual change started or that the action repeated in a certain period. 明るくなる or 貧しくなる are an example of the gradual change. (edit)
As you may know, た form doesn't only stand for past tense but also perfect aspect. So, 空が明るくなってきた can mean something like "the sky has just started to get brighter", which can be said in the same situation as "the sky is getting brighter". That's why you may find it translated that way.
Thinking of "the sky is getting brighter", the focus of this sentence lies, I believe, in the course of gradual change, which can be expressed with 明るくなってきている after all. Difference between 明るくなっていってる? Maybe "our sky" and "their sky" then.
Edit: ている stands for either a progressive action or a remaining result.
When てきている means the former, the difference between てきた is relatively simple and it focuses on how it's in the middle of continuous change, while てきた focuses on how the change started just before (edit).
On the other hand, when it means the latter, the difference is subtle in examples of gradual change (edit). If you take examples of repetitive actions, as is often the case with this usage, it's like the below.
- それまでやってきた: one
had repeated something until then
- 今までにやってきている: one
had repeated something before and the result reflects in the current time (one has done it by now)
- 今までやってきた: one has repeated something until the current time
てきていた is past tense of the above.
- 暖かくなってきていたころ: when it was getting warm
- 出会ったときにはすでに多くの成果を上げてきていた: He had already earned tons of successes when we first met.