Yes. SMBX 1.4 is not an older but alternative version of SMBX, made by 5438a38a
Most people here on the forums use X2, but there's no problem on using 1.4, you can use the version that you prefer
Depends what you mean by "1.4". If you mean SMBX-38A, then yes. If you mean version 1.4 of SMBX-38A, then no. The current version of SMBX-38A is 1.4.5, with many others also maintaining use of 1.4.4 for compatibility reasons.