Ethernet – Do straight-though and crossover still have any play in today’s business

Your thought process is based on faulty assumptions. By splicing 1 to 3 and 2 to 6 (which is what you have done), this does not put them in the same collision domain. Nor does this cause the devices to autonegotiate into half-duplex mode. Connect only one side of your main cable to one device and what you have now created is basically a "loopback."

Look at a hubbed 10BaseT or 100BaseTX environment. While they are in the same collision domain, they still have separate TX and RX pairs, they don't transmit in both directions on the same pair.

The reason for the collision domain is that a hub is ultimately a repeating device, which takes a signal in one port and repeats it out all other ports. If it receives traffic on two ports and repeats it out the other ports, the signal that is repeated will be a "collision" of the two signals creating an unusable signal. Thus the need for CSMA/CD and half-duplex operation.

By having three devices all spliced together in this fashion, you have merely created an environment that won't work. If you want to do this, simply put a hub between your two devices and connect a "listening" device to it. No need for a tap.

An uplink port is a port on which Transmit and Receive are reversed.

They are used to connect together 2 switches with a standard straight-trough cable. (otherwise it would require a cross cable where the transmit and receive are crossed in the cable rather than on the switch port)

Some switches came with 2 physical ports that were actually the same logical port , the first one being wired normally and the second wired "crossed". So you were able to use only one of the physical port a a time, depending if you connect a PC (or a router) or a switch.

Nowadays this generally doesn't matter anymore since most interfaces are "auto-mdix", meaning that the interface detect the type of device connected and cross transmit and receive internally if needed.

(and if the switch itself is not auto-mdix but the device you connect on it is , it also works).

Note: this apply mostly to 10Base-T / 100Base-TX ports, since Auto MDI-X is nearly ubiquitous on gigabit Ethernet, and doesn't apply to 10GB Ethernet and above where there's no more dedicated transmit / receive wires.