-very brief and disorganized notes (not a full answer), maybe someone else will be willing to write a more coherent answer-
Weiss 2020: 527
"Primary stress on the initial syllable is inferred on the basis of the syncope that affects medial syllables"
also see Lindsay 1894 pp. 157-160 (e.g. word-initial stress in facilius and mulierem in the plays of Plautus and Terrence, cf. Sommer and Pfister 1977: 74 fắcĭlĭŭs, sĕ́quĭmĭnī, cĕ́cĭdĕrō, mŭ́lĭĕrĕm; alliteration, e.g Meiser 1998/2010: 53 Cato Agr. 141,2f. "fruges frumenta vineta virgultaque ... pastores pecuaque salva servassis"); also see Leumann 1977 §243 (pp. 246-248) and esp. Sommer and Pfister 1977 pp. 73-74 for a very informative review.
Clackson and Horrocks 2007/2011 (pp. 47-48 and esp. 92-94)
Latin: The Lapis Satricanus (around 500 BCE) shows unweakened forms, their example is Mamartei, which they claim is supported by the other Latin inscriptions of the 7-6th centuries (they don’t mention which inscriptions though);
Etruscan (not an Italic language!): the first half of the 5th century (cf. Weiss 2020: 118, ft. 17 "around 470 BCE", "beginning at the end of the 7th century", p. 527), their example is Avile (6th century, Weiss 2020: 527) or Avele (6th/5th century, Weiss 2020: 527) in the earlier texts, whereas Avle in the later texts (for Aulus);
Sabellian languages: they write that “In texts written in the Oscan script long vowels are sometimes written with a doubled vowel sign […] found (with one exception) in word-initial syllables, suggesting a maintenance of vowel length under the word accent, but loss elsewhere” (p. 47)
Syncope of short medial vowels – between 6th-4th centuries, their example is Peracis (ca. 500 BC) vs. perkium (from a later Oscan text)
They also discuss an interesting case, destr- in Oscan and Umbrian vs. dextr- in Latin (cf. Greek δεξῐτερός). So they write that “we can know that the change which led to the similarity between Latin dextr- and Sabellian destr- took place when they were separate languages” (p. 48).
also cf. Umbrian mersto-'iustus' vs. Latin modestus (Sommer and Pfister 1977: 73)
“The chronology is unclear; to be sure, it precedes the syncope of word-internal short vowels” (p. 748);
‘An argument for at least a Proto-Sabellic date is the fact that in Oscan as well as in Umbrian the outcome of PI *ẽ (< PIE *n̥) is different, depending on its position within a word” (p. 748),
PI *ẽ (< PIE *n̥) > an (word-initial), en (elsewhere)
Oscan anter, Umbrian ANDER (Latin inter) vs. Umbrian iveka IUENGUA /iwengaf/ acc.pl. ‘heifers’, Latin iuvenca (p. 750).
C2, "probably a diffused trait" (p. 207);
either a development of Proto-Italic or an innovation that "arose in one of the languages of the Italic cultural Koine" which "spread across pre-existing linguistic boundaries" (p. 118, footnote 17).
also see Kurylowicz 1958 (L'accentuation des langues indo-européennes), pp. 381-384, for some interesting observations
also see Nishimura 2008 (a Ph.D. dissertation, UCLA) or Nishimura 2014 and Brent Vine (e.g. Vine 2012); Pultrova