[trechriron]

This is going to sound counter-intuitive to the standard scientific brain power on this forum, but...

Why worry about it?

Start at the end. You have a terraformed Venus laid out the way you envision. It has the remnants of Titan as a moon. Give it a couple more moons for dramatic flavor.

It has the rotation you want, the mass you want, the gravity you want, the weather you want, the seasons you want and the long lost remnants of humanity live there. Just like you want.

How did it come to be? Doesn't matter. It's a low tech fantasy world. Even the brightest scientists are only going to be able to hypothesize or dream of the technology capable of pulling all that off.

What do you want the adventures to focus on? Is scientific accuracy going to play a part in the campaign? What cool things do you WANT to happen on this long lost terraformed Venus? Regular meteor showers? Strange solar storms? Gravity shifts?

I would focus more on what you want and detail that out and not worry so much about the science.

You can also label your setting with a disclaimer... "no scientists were harmed in the making of this setting". :D