Wǒ zài jiā zuò zǎofàn, yǒu niúnǎi hé miànbāo.

Here 在 marks location: 在家 = “at home.” It is not the progressive marker here.
To show the progressive (doing right now), use:

• 我在做早饭。 (I’m cooking breakfast.) — 在 directly before the verb
• 我正在做早饭。 — stronger “right now” Combine location + progressive like:
• 我在家正在做早饭。
• 我在家做早饭呢。 (呢 softens and implies “in the middle of”)

Avoid both. Natural options:

• 我在家做早饭。 (neutral; context decides time)
• 我在家正在做早饭。 (both place and “right now”)
• 我在家做早饭呢。 (colloquial “I’m cooking now at home”)

Chinese doesn’t have obligatory tense marking. Without markers, 我在家做早饭 can describe a habit or a present action.
To mark completion:

• 我在家做了早饭。 (I made breakfast at home — completed) To mark experience:
• 我在家做过早饭。 (I’ve cooked breakfast at home before)

Here 有 is existential: “there is/are.” The location/topic is implied by the previous clause (“at home”). You could spell it out:

• 家里有牛奶和面包。 If you say 我有牛奶和面包, it means “I possess milk and bread,” which slightly changes the focus.

Not when speaking generally: 有牛奶和面包 is fine.
If you specify quantity, add a classifier:

• 牛奶: 一杯/一瓶/一盒牛奶 (a cup/bottle/carton of milk)
• 面包: 一片面包 (a slice), 一块面包 (a piece), 一条面包 (a loaf) Using 一个面包 is okay for a bun/roll; for a loaf, prefer 一条.

和 most naturally links nouns: 牛奶和面包.
To connect clauses, use words like 而且/并且/也/还:

• 我在家做早饭，而且/还有牛奶和面包。 For adding another item as an afterthought, 还有 is very common: 有牛奶，还有面包。

It’s ambiguous without time/aspect. Context decides:

• Habit: 我常常在家做早饭。
• Right now: 我在家正在做早饭。/ 我在家做早饭呢。
• Future plan: 我明天在家做早饭。/ 我明天会在家做早饭。

Both mean “breakfast.”

• 早饭 is very common in Mainland speech.
• 早餐 feels a bit more formal/written and is frequent in Taiwan and on signs/menus.
  Similarly: 午饭/午餐 (lunch), 晚饭/晚餐 (dinner).

Use 喝牛奶 (drink milk) and 吃面包 (eat bread).
If you want to be explicit:

• 我在家做早饭，喝牛奶、吃面包。 Using 有牛奶和面包 neatly avoids choosing between 吃/喝.

Yes, the comma is fine to join two related clauses with the same setting. You could also write two sentences:

• 我在家做早饭。家里有牛奶和面包。 For listing 3+ items, use the listing comma 、 and optionally 和 before the last item:
• 有牛奶、面包和鸡蛋。

Typical order: Subject + Time + Place + Verb + Object.

• 我今天早上在家做早饭。 Time usually comes before place. Avoid 我在家今天早上做早饭.

Use 没有:

• 我在家做早饭，没有牛奶和面包。 (I’m making breakfast at home; there isn’t any milk or bread.) To emphasize each item:
• 没有牛奶，也没有面包。

Yes. 还有 highlights addition:

• 有牛奶，还有面包。 (There’s milk, and there’s also bread.) It can feel a bit more conversational or emphatic than just 和.

Yes, Chinese allows topic–comment structures:

• Emphasize what’s being made: 早饭我在家做。
• Emphasize location: 在家我做早饭。 (less common than 我在家…, but possible for focus) These are stylistic; the neutral version remains 我在家做早饭.