Isaac is said to have died in 1856, but surely it was 1846 because not only does Elgiva appear without him in the 1850 census, she stopped having children ca. 1843/4.

Elgiva is a form of Olga/Helva, so "Elgiva" is not a mistake, but "Elgida" is.  The wife of Edmund I, King of Mercia, was named Elgiva.

— born in Montgomery Co., MO:
2.  Sarah E. DAVIS, b. 1830/1
3.  Joseph DAVIS, b. 5 Mar 1833 or 1835-40 
4.  Matilda DAVIS, b. 1835/6
5.  Mary DAVIS, b. 1838/9
6.  John DAVIS, b. 1841/2
7.  Martha L. DAVIS, b. 1843/4

1.  Family Group Sheet compiled by Georgie Tolleson and Betty Ann (Matthiesen) Cogliati.

2.  1830 Census Index and Digital Images (online at Ancestry.com; Image #5-6 of 8; extracted by Diana Gale Matthiesen):

1830

MO

Montgomery Co.

Upper Loutre Twp.

p. 214

Ln. 5

Isaac Davis

100 001 - 000 010

010 000 - 000 000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	4 or under	1825-1830	= Isaac, Jr. (b. 1827/8)
1 male	30-39	1790-1800	= Isaac (b. 1794-1800)
1 female	20-29	1800-1810	= Elgiva (b. 1805/6)
1 male slave	10-23	1806-1820	= name unknown

Listed one line below Elgiva's father, Isaac VAN BIBBER. 

3.  1840 Census Index and Digital Images (online at Ancestry.com; Image #1 of 13; extracted by Diana Gale Matthiesen):

1840

MO

Montgomery Co.

Dannelle Twp.

pp. 202B-203A

Ln. 29

Isaac Davis

101 000 100 - 210 001 000

no slaves

0100000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	4 or under	1835-1840	= Joseph (b. 1833)
1 male	10-14	1825-1830	= Isaac, Jr. (b. 1827/8)
1 male	40-49	1790-1800	= Isaac (b. 1794-1800)
2 females	4 or under	1835-1840	= Mary (b. 1838/9) = Matilda (b. 1835/6)
1 female	5-9	1830-1835	= Sarah (b. 1830/1)
1 female	30-39	1800-1810	= Elgiva (b. 1805/6)
1	person employed in agriculture

Listed two lines below James ESTILL, husband of Elgiva's sister, Mathilda. 

4.  1850 Census Index/Images (online at Ancestry.com; Image #88-89 of 272):  District No. 8, Boone Co., MO, pp. 372B-373A, 648/648, enumerated 14 Sep 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):

Elgida Davis	44	m			720	Missouri
Sarah E  "	19	f				"
Isaac V  "	22	m		Farmer		"
Joseph   "	17	m		Farmer		"
[next page]
Matilda Davis	14	f				Missouri
Mary      "	11	f				"
John      "	8	m				"
Martha L  "	6	f				"

Listed four households from John C. DAVIS. 

5.  1860 Census Images (online at Ancestry.com; Image #8 of 46):  Rockyfork Twp., Boone Co., MO, p. 644, PN 8, 50/50, enumerated 7 Jun 1860, official enumeration date 1 Jun 1860 (extracted by Diana Gale Matthiesen):

Isaac V Davis	33	M		Farmer	1600	700	Mo
Elgiva    "	54	F					"
Mary      "	21	F					"
John      "	18	M					"
Martha L  "	15	F					"

Listed next to John D. FRENCH.

6.  William F. Switzler.  1882.  History of Boone County, Missouri.  Western Historical Co., St. Louis, MO.  On p. 1107, under the heading, "EARLY HISTORY":

Isaac DAVIS came from Christian county, Kentucky, to this part of Missouri in 1818, and settled two miles east of Hallsville.  His sons were James, Henry, Isaac, Benjamin, John and Joseph.  One of Isaac's [Jr.'s] sons, Joseph, now lives on the old DAVIS homestead.

7.  Logan and Van Bibber Families by Bruce Logan.

8.  Broderbund.  World Family Trees.  Vol. 13, Pedigree #2579, submitted by Lawrence T. Pippin.

9.  LDS.  Family Search: Internet Genealogy Service: International Genealogical Index (IGI).

10.  Emails from Mary Clark, Harold Raymond Wilcox, Harman Reed Clark, Jr.