I visited this cemetery on 7/10/15 and only few stones remain but I found a list online and then received a copy of Tombstone Inscriptions Warren County, Illinois Hale Township Pleasant Green Cemetery from Warren County, Illinois Genealogical society from D.Webb. I have added everyone from that list with the info contained in that document. If there isn't a stone attached to the memorial then there wasn't one still standing.

Visited and further explored cemetery on 12/11/15 with my sister and we found more stones now that the grass was dead and trees almost empty so now adding more pix online

Found his will online and he later married Charlotte
His kids listed are John H., William, Elizabeth, Alexander M., and Zachariah

Thanks to Rex Strother #49684880 for providing 1837 as their year of marriage 10/10/20.
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All below information provided by Diane L. Teague Webb FAG47297921

Source:
Genealogical Data From The "Oquawka Spectator" Vol. II.
Henderson Co., IL. pub. 2004
by Mary Lozier page 206

Oquawaka Spectator June 5, 1884
Fought in Black Hawk War- buried in Pleasant Green Cemetery
Hardy M. Allen

Illinois, County Marriage Records, 1800-1940
Hardy M. Allen married 20 April 1837 Warren Co., IL
to Jane Monteath. (Their children Death Certificates say mother's
Maiden name spelled Montieth.)
2nd marriage:
Hardy M. Allen m. Charlotte Morgan 7 Feb. 1855 in Knox Co., IL.

1850 Federal Census
11 N 4 W, Henderson Co., IL.
Hardy M. Allen 43 Farmer 1700 Tenn.
Jane Allen 34 PA
John Allen 12 IL
William Allen 9 IL
Elisabeth Allen 5 IL
Alexander M. Allen 3 IL
Zacariah Allen O IL
David Reese 17 Laborer IND

Illinois, Wills and Probate Records 1772-1999
Hardy M. Allen
Probate 9 Apr 1862
Henderson Co., IL
Inferred Death Year abt. 1862
Death place: Henderson Co., IL
Probate Records, Vol A-C 1850-1905
Names wife Charlotte and 5 children in Probate.