With 100 Ohm line driver in the middle sees two parts in parallel, so effectively 50 Ohm. Typical driver impedance is about 40 Ohm, so while mismatched, mismath is not too bad. Also, with multiple devices on the line there will be undesirable signals even if you have termination at both ends.
In unterminated line there will be some loss, so after each reflection reflected signal will be weaker, in rough approximation multiplied by some number a < 1 (say 0.8). After n reflections signal will be multiplied by a^n and for large enough n will become negligible. Termination at given end with 1% resistor means that about 2% will be reflected (due to imperfection). This 2% is likely to be negligible. If transmitter is in the middle, there is still reflection at the end opposite to termination and at the transmitter. But mismatch at transmitter is not bad and the corresponding parameter a is much smaller than in unterminated case. So termination at one end reduces number of problematic reflections probably about 2-4 times. Which means that you can increase transfer rate by similar factor. Of course, termintion at both ends is better, but in multidrop case speed will be lower than in point-to-point link.
Well, multiple receivers on RS-422 have limited usefulness (AFAIK your use case is called 4-wire RS-485), so no wonder that FTDI does not support it. Maybe they have something more expensive that is doing what you want.
That is general thing, not specific to RS-485. If RS-485 receiver puts 24 kOhm load on line, that is about 0.4% of line impedance. When signal passes past receiver there is corresponding power loss. There is also second effect: receiver created discontinuity, so there is reflection. And beside resitive part receiver impedance has also reactive part which means that discontinuity and reflection is bigger than implied by receiver resistance. With lower load recevier effect is smaller, but still there is fraction of percent lost or reflected. Single loss is "very slight", but they add up and increase effective line loss: with single receiver reflecting/losing
0.5 after 40 receivers 20% of signal is gone. This 20% effectively adds to normal line loss.You probably should check if you can get such rate with short messages. If did little experiment using CH340 and CP2104. That was bi-drectional TTL level serial connection using 15 cm wires. Slave echoed each received character after mangling it a little (so I knew that it really came from the slave and not from some echo in software stack). I had trouble running CH340 above 460800 (that could be limit of program that I used). But using 1 character messages 10000 round trips took about 7s, with small influence from serial speed (almost the same result at 115200 and 230400). Also increasing message to 5 bytes gave essentially the same number of _messages_.
CP2104 was better, here I could go up to 2000000. Using 5 byte messages 10000 round trips needed 2.5s up to 1500000, at
2000000 time dropped to about 1.9. When I increased message to 10 bytes it was back about 2.5s.I must admit that ATM I am not sure what this means. But this 2.5s looks significant: this means 4000 round trips per second, which is 8000 messages, which in turn is number of USB cycles. So, it seems that normally smallish messages need USB cycle (125 uS) to get trough USB bus. It seems that sometimes more than one message may go trough in a cycle (giving smaller times that I observed), but it is not clear if one can do significantly better. And CH340 shows that it may be much worse.
FTDI is claimed to be very good, so maybe it is better, but I would not count on this without checking. Actually, I remember folks complaining that they needed more than millisecond to get message trough USB-serial.
OTOH, your description suggest that you should be able to do what you want with much smaller message traffic, so maybe USB-serial speed is enough for you.
You may be missing fact that most folks installing network cabling do not know about transmission lines and reasons for matching pairs. And even for folks that understand theory, it is easier to check that colors are in position prescribed in the norm, than to check pairs. So, colors matter because using colors folks can get correct connetion without too much thinking. Why two specs? I think that this is artifact of history and way that standard bodies work. When half of industry is using one way and other half is using different but equally good way standard body can not say that one half is wrong, they must allow both ways.