Learn Hardcore Japanese: kenkou no tame ni, satou ya sio wo iresuginai you ni ryouri wo kuhuusite imasu. - For my health, I try to cook in a way that doesn’t use too much sugar or salt.

AとB – usually lists items completely and exactly: “A and B (and that’s all).”
AやB – lists items non‑exhaustively: “A, B, and things like that.”

So 砂糖や塩 suggests:

“sugar, salt, and similar things (seasonings)”

If you used 砂糖と塩, it would sound more like only sugar and salt, as a complete list.

How is 入れすぎない formed, and what exactly does it mean?

The base verb is 入れる (“to put in / add”).

There is a pattern:

verb stem + すぎる → “to do (something) too much / excessively”

So:

入れ (verb stem of 入れる) + すぎる → 入れすぎる = “to put in too much”

Then we make it negative:

入れすぎない = “don’t put in too much / not put in too much”

In the sentence, it’s part of the phrase 入れすぎないように – “so that I don’t put in too much.”

What does ように do in 入れすぎないように?

よう is originally a noun meaning “way / manner / state” and ～ように has several uses.
One key pattern is:

V-ないように = “so that (something) does not happen / in order not to V”

So:

入れすぎないように
→ “so that I don’t put in too much”
→ “in order to avoid putting in too much”

It expresses the aim or desired result of the action that follows (here, the way you 工夫している your cooking).

Why does the sentence have two に’s: ために and ように? Do both mean “to”?

Yes, both ために and ように use に, but they belong to different grammar chunks:

健康のために – “for the sake of health / in order to be healthy” (overall purpose)
入れすぎないように – “so that I don’t put in too much” (more specific aim / condition)

They stack like this:

(For my health), (so that I don’t put in too much sugar or salt), I’m being creative with my cooking.

So you’re giving a big general purpose (健康のために) and then a more concrete strategy (入れすぎないように).

What exactly does 料理を工夫しています mean? Why not just 料理しています?

料理する by itself means “to cook / to do cooking.”
工夫する means “to devise / to be inventive / to think of clever ways.”

The expression 料理を工夫する means:

“to be inventive with one’s cooking”
“to find creative ways to cook / to cook with special care or ingenuity”

So:

料理しています – “I am cooking / I cook.”
料理を工夫しています – “I put thought into how I cook / I devise ways of cooking (e.g. to make it healthier).”

The extra nuance is that you are adjusting and thinking about how you cook, not just performing the act of cooking.

Why is it 料理を工夫して (with を) instead of something like 料理で工夫して?

工夫する usually takes the thing you’re being inventive with as a direct object marked by を:

時間を工夫する – to use your time cleverly
勉強方法を工夫する – to devise a good study method
料理を工夫する – to be inventive with your cooking

Using で (料理で工夫する) would be more like “to be inventive by means of cooking,” which is not the intended meaning here.
Here, 料理 itself is what you’re devising ways for, so を is correct.

What nuance does ～ています have in 工夫しています? Is it present continuous or habitual?

～ている (here: ～ています) has several aspectual uses. In this sentence it expresses a continuing, habitual practice:

工夫しています = “I (regularly / habitually) put thought into it / I am making ongoing efforts to be inventive.”

It does not just mean “I am at this very moment devising” (though in some contexts ～ている can be strictly present continuous).
For regular efforts or habits, Japanese often uses ～ている, where English would use simple present:

毎日走っています。 – “I run every day.”
健康のために工夫しています。 – “I make efforts for my health.”

Who is the subject of this sentence? There is no “I” or “we.”

Japanese often omits the subject when it’s clear from context.

In this sentence:

(私は) 健康のために、砂糖や塩を入れすぎないように料理を工夫しています。

The implied subject is typically 私 (“I”), or possibly うちでは (“in our household, we...”) depending on the context where it appears.

Because the action is something people usually do for themselves, the default natural reading is “I (am)…” unless previous context clearly points to someone else.