I wonder whether Helena was really born in Iowa, whether she was really their child, and whether she really died bef. 1870.

1.  Marriage Index:  Indiana, 1851-1900 (Broderbund CD-243):  not found.

2.  1860 Census Images (online at Ancestry.com; Image #268 of 355):  Auburn P.O., Butler Twp., De Kalb Co., IN, p. 268, PN 16, 109/112, enumerated 18 Jun 1860, official enumeration date 1 Jun 1860 (extracted by Diana Gale Matthiesen):¤•

Henry Stroup	30	M	Farming	100	Ohio
Mary W.   "	2?	F			New York
Solon H.  "	4	M			Ind
Helina M. "	2	" (sic)			Iowa

Yes, male, and yes, Iowa.  Living next door to Henry's parents, Joseph & Elizabeth (MILLER) STROUP. 

3.  Library of Michigan 1870 Census Index and Images (online at the Library of Michigan [link died]) and 1870 Census Index/Images (online at Genealogy.com; Image #34 of 36):  Dowagiac P.O., Pokagon Twp., Cass Co., MI, p. 181B, Roll 668, PN 34, enumerated 17 Jun 1870, official enumeration date 1 Jun 1870 (extracted by Diana Gale Matthiesen):¤•

1870:  for an explanation of the column headings, please see What the Numbers in the Federal Census Mean (missing columns contained no data).
1	2	3	4	5	6	7	8	9	10	15	19
267	275	Straub Henry	40	M	W	Farmer	3000	500	Ohio		/
		______ Mary	36	F	W	Keeping house			New York
		______ Solon H	14	M	W				Indiana	/
		______ Joseph W	6	M	W				Indiana
		______ Willie L	2	M	W				Michigan

Our subject is the only Henry STRAUB/STROUP in Michigan.

4a.  1880 United States Census and National Index (LDS Family History Resource File: CD-ROM Library):¤•

Census Place: Pokagon, Cass, Michigan
Source: FHL Film 1254575; NARA Film T9-0575; Pg 560A
	Relation	Sex	Marr	Race	Age	Birthplace	Occupation	Fa	Mo
Henry STRAUB	Self	M	M	W	50	OH	Farmer	PA	PA
Mary W. STRAUB	Wife	F	M	W	46	NY	Keeping House	NY	NY
Solon H. STRAUB	Son	M	S	W	24	IN	Works On Farm	OH	NY
Joseph W. STRAUB	Son	M	S	W	16	IN	Works On Farm	OH	NY
Willie L. STRAUB	Son	M	S	W	12	MI	At Home	OH	NY
Richard H. STRAUB	Son	M	S	W	5	MI	At Home	OH	NY

The only other adult Henry STRAUB/STROUP in Michigan is the one who married Anna MEYER. 

4b.  1880 Census Every-Name-Index/Images (online at Ancestry.com, Image #47 of 82):  Silver Cliff, Custer Co., CO, Roll T9_89, p. 276C, PN 47, SD 12, ED 32, enumerated 21 Jun 1880, official enumeration date 1 Jun 1880 (extracted by Diana Gale Matthiesen):¤•

1880:  for an explanation of the column headings, please see What the Numbers in the Federal Census Mean (missing columns contained no data).
1	3	4	5	9	13	24	25	26
70	Straub Solen H	W	24	/	Miner	IN	OH	OH

It appears Solon got caught up in the "silver rush" to Colorado and managed to get himself enumerated twice.  There are notes in the margin that I cannot read, but he is one of many miners (single men and married men without their families).

5.  Family Group Sheet No. 10856 (Everton Publishers, Inc., P.O. Box 368, Logan, UT 84323-0368; Broderbund CD-14).

6.  Messages posted to the STROUP Surname Board (online at Ancestry.com/Rootsweb.com).