I tend to agree with you. I believe Irene Prudence McGinnis' father is Phineas McGinnis, probably, but not proven, brother of Samuel. The 1830 census of Johnson County, IL are 5 McGinnis families, Mary is the oldest and probably the widowed mother of the other 4 men, Henry, William, Samuel, and Finis. I cannot find Finis in any other census record. A popular name with this family, and the Gibsons that Irene married into is Phineas. Indeed, Mahala's eldest son in 1850 is Phineas. If Samuel McGinnis is living in 1850 in MO, then Mahala's spouse is NOT Samuel. I chose to believe it is Phineas. Finis McGinnis of the 1830 Johnson CO IL census has only 2 children, both age<5. This would support a marriage date of 1825-1827.
And, Mahala is equally a mystery. There is a marriage record for Finius McGinnis and Rachel Evans in Union County, IL, 16 Jul 1826. Union is a neighbor county to Johnson. So is Mahala McGinnis of the 1850 census Mahala Rachel Evans, Rachel Mahala Evans, or Mahala Bridges? Did Rachel die early and Phineas remarry? Indeed, in the 1830 Johnson CO IL census, Samuel McGinnis is living next door to 3 Bridges families, Alfred, Calvin, and John Jr. There is a John Bridges in the 1820 Johnson CO census, age>45. I think this is the main reason that some think Irene's mother is Mahala Bridges. I cannot prove it either way, but it's hard to refute a civil marriage record. Most family trees assign Rachel Evans as the wife and have the name of Mahala either first or second.