1- its very true if they can get you to your Destination earlier then you would have they need not give you anything!Especially if they are FIMing you before boarding is complete.

That said when they hit me with that I simply answered OK I will stick with my original routing, they then offered a $200 voucher I said , I guess you didnt hear me I have no problems flying on my tkted flights and will do so , so please print the tag for my checked bag. She walked away came back a few mins later and said You win we need your seat and $400 it will be

2- Once You AGREED to whatever the offer was thats it anything after that with the exception if they just offered another person more who is in a similiar sit as you then they should match that , but honestly thats usually Only happenin at the Gate. otherwise its SOL

3- Once you were sent to US you now became tehir problem as long as you were in fact tkted for that flight and UA need not deal with You again till your return flight if there was 1. Its now USs Problem to get you to CMH and yes if UA agrees they can FIM you back to UA.But once US accepted you,you no longer had a UA tkt but a US tkt. Could be UA has ways to CX all the FIMing but US would have to return the tkt back over to them and I believe you would ahve to Agree to that.

4- the rest is a commedy of errors and UA may throw a small bone your way but dont expect much and no they dont owe you the $200 VDB. Once they said no VDB you could have told them as I did above Thanks but No Thanks instead you ACCEPTED their terms. taht ends that unfortunately

sometimes things go very smoothly other times it all hits the fan at once, unfortunately for you it all hit at once