With 4 threads I will probably be able to check 240 blocks per second but many people have much slower processors. With your proposal physics simulation will become much slower and to really harm the bots we will need to make it at least 100 or 1000x slower. This will make block checking very slow, on some computers it will take several seconds to check one block, some probably would check new blocks slower then they are generated, inital synchronization with network will take days or even months, botnets will perform spam attacks sending a lot of invalid blocks and nodes will use 100% of their CPU time to check these blocks.
Yup. Not to mention the "clincher" here which is that the bots could simply run their solutions without this "added legwork" and then re-run their path against the "full physics" once a solution is found to verify that it will be accepted by the network. This would just hamper normal users a LOT and the bot heuristics very very little.
I think it's not true, this is the type of game in which every minor difference in speed or position can lead to huge and unpredictable trajectory changes, so bot will have to use real physics and not "fake" one.