1 i have been doing these walls for 30 years.
Pouring concrete under existing wall.
2 i have tested several types of bags.
Once the truck has reached the end of a section spread the concrete out evenly and a touch higher than the form with a concrete placer rake.
Pour the concrete in small sections spreading the concrete by moving the chute back and forth and by having the driver pull forward as you go.
This may require jacking up the car port to accommodate.
I would also pour concrete higher than the existing deck in order for you to slope the top of the pour i m order to shed water.
4 i can disprove your theory that the concrete will not set up correct due to water ratios.
3 that video above with the guy putting steel in the bags is my video.
Pour an interior wall.
A professional foundation crew will construct a one sided concrete form inside the old basement perimeter and pour a new concrete wall.
Deck footings closer than 5 to an existing exterior house wall.
Footings under existing slab.
In this video we take you through the basics of how to remove set up and pour a concrete room with in your own home.