The arc of a rainbow has nothing whatsoever to do with the alleged flat earth dome. Even if the dome were real (which it is not), rainbows would be completely unrelated.
In your pictures, no. To the naked eye from the perspective of the earth's surface, also no, because the human eye cannot discern whether a horizon is flat or curved as the degree of curvature is beneath the level of perception. That doesn't preclude the actual curvature of the very large earth.
The closest English wording for "only begotten" is "species-unique" or "one of a kind". It does not mean "fathered" (as of biological procreation). I give credit to Dr. Michael Heiser for this research-based view.
A bad marriage is the result of sin. There are marriage-ending sins, and there are many more sins that need not end the marriage. Don’t blame God for human sin, and don’t think that He can’t redeem even a terrible marriage.
Welcome to CC! I’m a step-parent too. Some years back I took a course offered through Focus on the Family on the subject of blended families. ‘Blended and Blessed’. You may want to check it out.