Exceeding max front difference & 9-speed MTB / Road (in)compatibility
 

I'm planning a new (first) bike build and it's going to be a do-everything bike likely a conversion of an existing hybrid to drop bar with somewhat customised gearing. I've got two questions...

1) I've read elsewhere that Shimano 9 speed MTB FDs are compatible with road shifters. Does that mean my Sora 3503 triple STI can shift a current generation 9-speed Shimano mountain triple FD (say Altus M370)?

2) The front chainset I've in mind is 26-36-48 which means a 22T difference. The Sora 3500 GS RD has a specified max front difference of 20T - should I expect a catastrophic crash or just minor rubbing?

No, Shimano 9-speed MTB rear derailleurs are compatible with road STI shifters. MTB front derailleurs are not compatible with road STI shifters.