Half-duplex base-T Ethernet does not mean that the same wires are used for both directions of transmission, only that the two ends cannot transmit simultaneously.
Full-duplex means that both ends can transmit and receive at the same time, which only became possible with the shift from the shared coaxial cable to point-to-point unshielded twisted pair cabling.
There are never any physical collisions with base-T Ethernet, only "logical" collisions within a hub or switch. Even with Gigabit Ethernet, in which all four pairs are used in both directions simultaneously, each end uses a "digital hybrid" to separate incoming data from outgoing data on each pair.
Different signalling rates are generally completely incompatible with each other. Both ends of the link have to be running at the same speed for the link to come up. Now, it's possible for one end to support faster speeds than the other end so long as they are either explicitly configured or they autonegotiate to a common rate. For example, if you connect a computer with a 100M ethernet card to a router with 1G ports, the link should autonegotiate to 100M and come up.
For straight serial signalling, the receiver has to lock on to the data stream. The way the receiver is built, this is only possible if the actual data rate is within a few percent of the expected data rate. Raw serial rate for 1G ethernet over fiber is 1.25 Gbps. Raw serial rate for 10G ethernet over fiber is 10.3125 Gbps. Generally 10G PHYs can be configured to run at 1.25 Gbps, but it is not possible for a deserializer configured for 10.3125 Gbps to lock onto a data stream running at 1.25 Gbps.
High speed serial links also use line codes to provide certain electrical characteristics, such as DC balance and guaranteed minimum transition density to aid in clock recovery. Different rates can use different line codes. 1G ethernet over fiber uses the 8b/10b line code. 10G ethernet over fiber uses the 64b/66b line code. You can't receive 8b/10b line code with a receiver configured for 64b/66b, even if the serial data rate is the same.
Ethernet over copper cabling has the same set of issues, but there can be very substantial electrical differences as well. 10 Mbit ethernet transmits on one pair with Manchester encoding to transmit the combined data and clock signal. 100 Mbit ethernet transmits on one pair with a 4b/5b line code and MLT-3 (multi-level transmit), transmitting with 3 different voltage levels. 1 Gbit ethernet uses some fancy echo cancellation signal processing to transmit on all four pairs in both directions at the same time with PAM-5 (pulse amplitude modulation), transmitting with 5 different voltage levels.
So as you can see, if the settings do not agree, the receiver is going to be horribly confused and no data will be transferred.
Best Answer
The answer to the question you referenced is correct... more precisely, it was correct until EEE (Energy Effective Ethernet) was invented. EEE employs the same fast link pulses to signal the link is up while the phys are in power-saving mode.
In 10BASE-T, FLPs are used to indicate the link is still on while there are no packet transmitted between the stations.
In 1000BASE-T, auto-negotiation is mandatory, you cannot turn it off at all, at least master-slave relation must be resolved from AN.
Also, it is preferable to not turn off AN even if you want to operate on an exact speed and duplex. You must select only that link ability (mode) and then start the AN process.
Therefore, my answer to this question is that the stated assumption is correct in some cases only, not in all.