It has to do with the IC process. If they make the devices the same size they will be different in on-resistance because one of them is p-channel. They could upsize to make it equal but chip real estate is a cost factor.
One thing to note is that many gate drivers cannot be put into a hi-Z state on their outputs which makes then not so attractive for RS485.
That's an advantage of being a consultant. I just roll up the sleeves and build it :-)
To be honest, I do not see any gradual decay. I'd be surprised if it happened because normally what matters is the parasitic device capacitance and that should be independent of any receive enable.
I like this one here:
:-)
It can work. The trick is to high-pass the feedback so only the sluggish transitions get spiffed up but the remainder of each high or low phase remains largely untouched. It requires brief current injections from a circuit that goes hi-Z after each injection. Use a blazingly fast buffer to keep the latency down.
Keeping all that from going berserk (oscillations, chain-bucking, stampeding) isn't a small feat but if it works you are the hero of them month.
Much of this can be simulated in SPICE but it will take time. It's not something you can draw up on a napkin while wolfing down a burger and be done with it.
It would need to run at several MHz at the least if you need 1Mbit/sec. That can become tedious if you must run that inside a uC with rather limited horsepower.
It doesn't even have to be PLL. You can send, for example, two frequencies around 10MHz but at least 4MHz apart. Lowpass and Schmitt this, then run the RX signal into a timer which counts the number of uC master clock cycles between transitions. Average a little for noise reduction and signal an error if this changes too often. The timer count would say whether a one or a zero was received.
This is kind of hard to explain. An open transformer has a certain primary inductance, which results in:
Z = 2 * Pi * f * L
where f is the lowest frequency you Manchester code presents to it. The old rule of thumb for point-to-point transmission is that you want Z to be 400ohms or higher for a 100ohm line. In you case you'd rather want it to be 10k or so. That isn't easy but may be possible. One way to start is to browse
For example, this series fulfills your 3mm height limit and gets you to about 1k at 1MHz or (theoretically) 10k at 10MHz:
Now the rather high interwidning capacitance might spoil the broth here but you could buy a few different kinds of such transformers and just try. It allows you to keep things as differential as possible.
LAN transformers might also work but you'd have to try it out:
Just an example, and this is a wider 4-channel. So you'd have to call them and ask for something smaller. Often you can latch on to a custom run. I have sometimes done that for the ferrite cores, sort of hitching a ride.
I don't see the height restriction being an obstacle here, as long as you can sprinkle parts around.