Hello James!
If you have an MTU of 1500 set on a port, then any frame that attempts to ingress on this port that is larger than 1500 bytes will be dropped. If the port is set up to accept larger frames, then it will accept them. As for the FCS, if a frame is smaller than the MTU size that has been configured (either jumbo or not) it will be dropped if the FCS fails regardless of size.
Note that the DF bit is on the Network Layer, that is, in the IP header. This bit tells the SENDER weather or not the IP packet can be fragmented during encapsulation. Let’s say a PC is encapsulating part of an email being sent. Say the IP packet is 1700 bytes and it is being encapsulated from layer 3 to layer 2 and the DF bit is set to 0 (allowed to be fragmented). The MTU of the NIC of the PC is set to 1500 bytes. The PC will split the IP packet into two, put one portion in one frame of less than 1500 bytes and the other portion in the next frame of less than 1500 bytes. Notice that the DF bit only plays a role at the SENDER. When this frame reaches the port of a switch, it will blindly check the size, and if it is too big, it will drop it. This is why it’s important that MTU be the same on both ends of a link. (I’ve had to learn this the hard way troubleshooting strange behaviour between two sites for almost two days, where some data goes through and some does not, seemingly at random!!)
Now if the DF bit was set to 1, then the encapsulation would not occur at all at the PC, and the sending would fail. This once again shows how the DF bit has to do with encapsulation at the SENDER.
I hope this has been helpful!
Laz