Would the schedule change be enough to justify a modification of the child support?

You must show that the current amount would change by twenty percent to meet the test.

Social security benefit taxes

The other attorney responses seem sufficient to me.

Can dad take my son to his attorney appointments with him to be coached prior to going to court?

Not sure who dad is. If grandfather only during his respective child parenting time.

Can I back out of a signed contract without having recieved any payment from people who do the "we buy houses"?

A contract may include termination provisions but I doubt it. Failing to meet their obligations may provide you an out.

It depends upon the court and how backed up they are or how efficient their clerks are.

Will child support obligation decrease with adoption of stepchild?

Perhaps. He must file a motion to modify and use that variable to argue a lesser amount.

Tax payment plan?

I'm unable to determine what your query is. Perhaps the other responses will pick up the slack.

Hire legal counsel to draft the documents.

Hire legal counsel and allow them to address the matter professionally.

Do I have to pay capital gains tax on my house i sold.

Sounds like you have met the two year requirement in Missouri. Check your state's minimum.