How can he have died in 1774 if he answered the alarm in 1775?
Barzillai is a scriptural name; other spellings are misspellings.

Which is wrong, Silas's birth year or baptism year?

1.  Index to Massachusetts Town Birth Records: Worcester County: Lancaster compiled by the New England Historic Genealogical Society (online at Ancestry.com).  Dates are not given in this index, so the original work must be sought.

Name	Sex	Birth Place	Father	Mother
Barzillai Holt	M	Andover	James	Susannah

2.  Frederic W. Bailey. 1897-1914.  Early Massachusetts Marriages Prior to 1800.  3 vols.  Bureau of American Ancestry, New Haven, CT (online at Ancestry.com):

p. 107

Barzillai Holt & Lois Allerd, Feb. 22, 1759

3.  Anon.  1923.  "Records of the Church in the North Parish of Shrewsbury, [Worcester Co.,] Mass.: from a Manuscript in the Possession of the New England Historic Genealogical Society."  New England Historical and Genealogical Register 77(January): 17-33.

p. 22	1750... Apr. 8 Berzillai Holt & his wife owned y Covenant & obtained baptism for yr. 3 childn, viz., Abel, Berzillia, & James... Aug. 26 Silas Son of Berzillai Holt baptized.  Item Br. Joshua Houghton recommended to the first Chh in Lancaster.
p. 23	1753... Sept. 23 Ezra son of Jonn. Ball, Elizabeth daughter of Barz. Holt, & Sarah daught of Daniel Bixbey baptized.
p. 27	1763... May 15  Abial Son of Bazillai Holt baptized.
p. 28	1765 Feb. 24 Jotham Son of Berzillai Holt & Lydia daughter of Micah Hathern Baptized.

4.  Daniel S. Durrie  1864.  A Genealogical History of the Holt Family in the United States: more particularly, Descendants of Nicholas Holt of Newbury and Andover, Mass., 1634-1644. Munsell, Albany, NY.

5.  Holt Association of America.  1930.  The First Three Generations of Holts in America. Moore Printing Co., Newburgh, NY.

6.  William Sweetzer Heywood.  1893.  History of Westminster, Massachusetts... Vox Poluli Press, S.W. Huse & Co., Lowell, MA (online at HeritageQuest at Ancestry.com):

p. 172	WESTMINSTER MEN IN THE REVOLUTIONARY WAR... NAMES OF SOLDIERS Who went to Cambridge upon hearing of the battles of Lexington and Concord, April 19, 1775, for ten days' service... CAPTAIN MILES' COMPANY, IN COL. JOHN WHITCOMB'S REGIMENT... Bezaleel Holt, corp. Silas Holt, fifer...
p. 177	ALPHABETICAL CATALOGUE Of Westminster Men known to have been in the Revolution. MINUTE MEN... Holt, Barzillai Holt, Silas...

7.  LDS.  Family Search: Internet Genealogy Service: Ancestral File (AF) and International Genealogical Index (IGI).  Various IGI records have them married in Lancaster, Marlborough, or Andover.  The AF has them married in Lancaster.