Learn Hardcore Japanese: kanozyo ha simekiri wo tyanto mamoru.

は marks the topic, so 彼女は means as for her. It frames what follows as a comment about her general tendency or the current discussion topic.
Use が when you want to mark or emphasize the subject (often answering who).

彼女は締め切りをちゃんと守る。 As for her, she meets deadlines properly. (neutral, descriptive)
彼女が締め切りをちゃんと守る。 It is she who meets deadlines properly. (contrastive or answering 誰が…？ who does…?)

Typical Q-A:

誰が締め切りを守るの？ — 彼女が守る。 (Who meets the deadline? — She does.)

Why is を used with 締め切り? Could I use に, or drop を?

守る is a transitive verb that takes a direct object, so the thing you keep/observe is marked with を: 締め切りを守る.

に is wrong here; use に with different verbs like 間に合う (be in time): 締め切りに間に合う.
Dropping を can happen in very casual speech, but it’s safer and more natural to keep it: 締め切りをちゃんと守る.

What exactly does 守る mean here, and what else can it take?

With 締め切り, 守る means to keep/observe/adhere to a deadline. Other common collocations:

約束を守る keep a promise
規則・ルールを守る follow rules
時間を守る be punctual
秘密を守る keep a secret

The opposite is often 破る (break/violate): 締め切りを破る, 約束を破る.

What nuance does ちゃんと add? Could I use きちんと, しっかり, or 必ず instead?

ちゃんと is a manner adverb meaning properly, reliably, as expected. It emphasizes doing something the right way.

きちんと is a bit more formal/neater in tone than ちゃんと and is safe in writing: きちんと守る.
しっかり adds a sense of firmly/solidly; with 守る, it can imply strictness or thoroughness: しっかり守る.
必ず means without fail/certainly; it stresses certainty, not manner: 必ず締め切りを守る.

In formal business writing, prefer きちんと, 必ず, or set phrases like 厳守.

Can I move ちゃんと around? Is 彼女はちゃんと締め切りを守る okay?

Yes. ちゃんと is a sentence adverb and can float within the predicate:

彼女は締め切りをちゃんと守る。 (neutral)
彼女はちゃんと締め切りを守る。 (slightly earlier emphasis on the manner)
With the subject omitted: ちゃんと締め切りを守る。 (common)

Avoid 締め切りを守るちゃんと (unnatural). Also, don’t say ちゃんとに; ちゃんと is already an adverb.

What tense is 守る here? Does it mean she always does this, or that she will?

Plain nonpast in Japanese covers:

Habitual/generic: she tends to do it.
Future: she will do it (context decides).

To clarify:

Habitual: 彼女はいつも締め切りをちゃんと守る。 (always)
Future specific: 彼女は今回の締め切りを守る。 (this time)

The progressive 〜ている can mark habit when supported by context/frequency words:

彼女は（いつも）締め切りを守っている。 (she habitually meets deadlines) Without such cues, 守っている may be read as currently complying with a specific deadline.

How do I make this more polite or business-like?

Polite: 彼女は締め切りをきちんと守ります。
Polite request: 締め切りを必ず守ってください。
Formal/business word choices:
- 期日（きじつ） due date, 納期（のうき） delivery date, 提出期限 submission deadline
- 厳守（げんしゅ） strict observance: 期日厳守でお願いします。
- 遵守（じゅんしゅ） comply with (very formal): 規則を遵守します。

Do Japanese actually say 彼女 for she? Could it mean girlfriend?

彼女 can mean she or girlfriend. In everyday conversation, third-person pronouns are often avoided; people prefer names or just omit the subject when understood.

Natural: （田中さんは）締め切りをちゃんと守る。
Using 彼女 is fine in writing or when there’s no risk of confusion, but in casual talk 彼女 often implies girlfriend.

How do I read and write 締め切り? Are there alternative spellings or words?

Reading: しめきり.
Usual spelling: 締め切り (noun). Variant 締切 is common in signs and headings.
Related forms: 締め切る (verb, to close the acceptance), 締切日 (deadline date).
Synonyms and near-synonyms:
- 期限（きげん） time limit/term (general)
- 期日（きじつ） due date (formal) Note: On doors or switches, 締切 can mean kept closed/shut, unrelated to deadline.

What other verbs go naturally with 締め切り, and how do they differ from 守る?

締め切りを守る honor a deadline
締め切りを破る miss/break a deadline (strong)
締め切りを過ぎる the deadline passes (time-focused)
締め切りに間に合う be in time for the deadline
締め切りに遅れる be late for the deadline
締め切りに間に合わせる make it by the deadline (through effort)
締め切りが迫る the deadline is approaching
締め切りを延ばす／延長する extend the deadline

What exactly does ちゃんと modify here? Can I say ちゃんとしている instead?

In 締め切りをちゃんと守る, ちゃんと modifies the verb 守る (the manner of keeping the deadline).
ちゃんとする means do properly; ちゃんとしている describes a state/character (proper, orderly). You cannot say 締め切りをちゃんとしている; but you can say 締め切りをちゃんと守っている (is properly meeting deadlines, with context).

How do I say she can meet deadlines?

Use the potential form of 守る:

彼女は締め切りをちゃんと守れる。 (She can meet deadlines properly.)

With potential verbs, the object may be marked by が in formal grammar, but を is very common in modern usage:

締め切りが守れる (also possible) vs 締め切りを守れる (very common).

You can also use 間に合う:

彼女は締め切りに間に合える is uncommon; instead say 彼女はいつも締め切りに間に合う (as a habitual statement).

How can I add always/usually/never to this sentence?

Insert a frequency adverb near the verb:

彼女はいつも締め切りをちゃんと守る。 always
彼女はたいてい締め切りを守る。 usually
彼女はめったに締め切りを守らない。 rarely (note the negative)
彼女は絶対に／必ず締め切りを守る。 without fail

What happens if I topic-mark 締め切り with は? Can I make a double topic?

締め切りはちゃんと守る。 As for deadlines, (I/She) keep(s) them properly. This puts contrast or focus on deadlines themselves.
Double topic for contrast is possible: 彼女は締め切りはちゃんと守るけど、メールの返信は遅い。 As for her, deadlines she keeps, but her email replies are slow.

How do I negate or put it into the past naturally?

Past: 彼女は締め切りをちゃんと守った。 She met the deadline (properly).
Negative: 彼女は締め切りを守らない。 She doesn’t meet deadlines.
With manner: ちゃんと守らない implies not properly (e.g., cutting it late or missing details).
Couldn’t: 彼女は締め切りを守れなかった。 She couldn’t meet the deadline.
Stronger miss: 締め切りを破った。 She broke the deadline.

Are the spaces in 彼女　は　締め切り　を　ちゃんと　守る。 normal?

No. Japanese is normally written without spaces. The natural writing is 彼女は締め切りをちゃんと守る。 The spaced version is just for teaching/segmentation.