You are confusing bitrates here, the fax emits an analog signal and doesn't interface the digital line directly- it's not a digital packet switch protocol. Tho

You are confusing bitrates here, the fax emits an analog signal and doesn't interface the digital line directly- it's not a digital packet switch protocol. Those 2400bps are not using 2400bps from the line, but are modulated to an analog signal that uses the lines full bitrate. From Nyquist–Shannon sampling theorem we know that a 56bps line can transfer something like half of it in analog form, so 33.6 is marginal. T.38 is sensitive since it needs to end the analog side, practically receiving the fax. Fax in general is sensitive since the standards have left too much freedom for implementers, especially like you said in the timing parts, causing incompatibilities between machines. (Source: I have built fax relay over VoIP products in the late 90's)