All I could do was pray.
I am simply astonished.
I am filled with nothing but gratitude.
I was just waiting for time to pass.
When I heard the news, I was simply overwhelmed with sadness.
全く means 'utterly/completely' and can modify almost anything, whereas ただただ specifically emphasizes that something is only and entirely one thing, often with emotional nuance like regret or gratitude.
Standard kana spelling; the word is usually written in kana.
Rare kanji form using 只; not commonly used.
Rare kanji form using 唯; not commonly used.
Reduplication of ただ (only, just) for emphasis. The exact historical derivation is straightforward; the kana spelling is standard.