Was Shaktism prevalent in Vedic times? Do Vedas praise the Supreme almighty Shakti?
I'm only answering this part of the question so consider this a partial answer:
Do Vedas praise the Supreme almighty Shakti?
Yes, the Vedas do contain hymns in praise of the Shakti. One such hymn is the Devīsūkta, also known as Aṃbhṛnīsūkta, in the Rig Veda:
I (Devi) proceed with the Rudras, with the Vasus, with the Ādityas and the Viśhvadevas; I support both Mitra and Varuṇa, Agni and Iñdra, and the two Aśvins. (Rig-Veda 10.125.1)
I support the foe-destroying Tvaśtṛ, Pūśan and Bhaga; I bestow wealth upon the institutor of the rite offering the oblation (havis) - (who is) pouring forth the libation and deserving of careful protection.(Rig-Veda 10.125.2)
I am the sovereign queen (of all Existence), the collectress of treasures, cognizant of (the Supreme Being), the chief object of worship; as such, the gods have put me in many places, abiding in manifold conditions, entering into numerous (forms). (Rig-Veda 10.125.3)
He who eats food (eats) through me; he who sees, who breathes, who hears what is spoken, does so through me; those who are ignorant of me, perish; hearken who is capable of hearing, I tell you that which is deserving of belief. (Rig-Veda 10.125.4)
I verily myself declare this which is approved of by both gods and men; whomsoever I choose, I render him an exalted one, make him a ṛṣi, make him Brahman or make him highly intelligent. (Rig-Veda 10.125.5)
I bend the bow (to fasten the bow string at its ends) for Rudra, to slay the tormenting, Brahman-hating enemy. I wage war against (hostile) men (to protect the praying ones); I have pervaded the Heaven and earth. (Rig-Veda 10.125.6)
I bring forth the paternal (heaven - abode of the manes) upon the crown (brows) of (this Supreme Being); my genesis is from the waters (cosmic - the all pervading Entity); from thence, I pervade through all beings and reach this heaven with my body. (Rig-Veda 10.125.7)
I verily myself breathe forth like the wind, issuing out form to all the created worlds; beyond the heaven, beyond the world (I Exist eternally - beyond space & time) - so vast am I in my greatness. (Rig-Veda 10.125.8)
According to Upendrakumar Das, one great scholar on Shaktism, the answer is Yes.
Some scholars think that all the principal forms of the Vedic Gods are forms of Shakti.Tge fact is the Devas are neither males nor females.
In Rigi-Veda, Aditi has been referred to as Parama Devata. We must mention the Ratri-Sukta and Devi-Sukta also. In Vrihaddevataa, Ratri has been referred to as Kaali.
Sinivali,Raakaa, Kuhu are the names of the Vedic goddesses.Vrihaddevata treats them as alternative names of Saraswati. Shree and Lakshmi have been both considered as alternative names of the same Devi. Saraswati, Vaak, Ila--tyese Vedic goddesses wewr forms of Saraswati, as mentioned in the Veda-Samhita. In Satapatha-Brahmana, Vakdevi takes the form of a Lioness.The Sankhyana-Grihyasutra mentions Sri and Bhadrakali simultaneously. Sukra-Neetisaara provides a four armed description of Saraswati.
The name of Amvika is found in the Sukla-Yajurveda. The name of Durga is mentioned in the Taittriya-Aaranyaka. According to yaska, Agni and Aditi are the same. In Rig-Veda, we get the name of Mudgalani.
The Sweataaswatara-Upanishad mentions names of Maya or Prakiti.(Ref: Bharatiya Saktisadhanaa om Saaktasaahitya, RMIC, vol 1, chapter 4)