The following are excerpts from the teachings of Sri Ramana Maharishi.
D.: Is not Grace the gift of the Guru?
M.: God, Grace and Guru are all synonymous and also eternal and
immanent. Is not the Self already within? Is it for the Guru to bestow
It by his look? If a Guru thinks so, he does not deserve the name.
The books say that there are ...
Yes, You are looking for Rigveda 1.32.1
The verse of Agnipurana you're talking about in the question might be Agnipurana 259.14 (259th chapter from Agnipurna is "Rigvidhanam") quoting from sa.wikisource.org: (Transliterated into IAST):
ṛgbhiḥ ṣoḍaśabhiḥ kuryyādindrasyeti dine dine |
hihaṇyastūpamityetajjapan śatrūn prabādhate || 259.14 ||
The question is: Can mantras help transfer energy miraculously to a person or object?
It was also queried in Edited version of the question, as follows:
The Source/Giver repeatedly transfers or transmits spiritual energy
for a good cause or destruction in war as may be intended or desired.
Have such miraculous events been in evidence in recent ...
Siddhis (Sanskrit: सिद्धि siddhi; fulfillment, accomplishment) are spiritual, paranormal, supernatural, or otherwise magical powers, abilities, and attainments that are the products of spiritual advancement through sādhanās such as meditation and yoga.
The King in question might have obtained the siddhi - Prāpti:, ability to be anywhere at will.
Short letter Mantras
Probably the shortest sukta in the Vedic literature is in Atharva veda. This sukta is just of single line giving the names of the chief Vedic metres also known as Chandansi sukta (छन्दासि सूक्त ) Atharva-Veda Book 19-Sukta 21.This mantra is just having 25 letters splitting the words in multiple letters.
गायत्र्यु १ष्णिगनुष्टुब् ...
Both the correct words and the mantra is mentioned in following sources.
National Institute of Indian Medical Heritage (NIIMH) - ebook
Indian National Science Academy Journal
It is called Dhanvantari Nighantu, that's all I could find in my own search for the correct mantra, which also led me to this question.
Edit: As I searched further about Moola ...