MIDI out / thru circuit questions
The official circuit uses three 220 Ω resistors (two in the transmitter and one in the receiver) to limit the current to about 5 mA.

Dropping one resistor will increase the current to about 8 mA. This is unlikely to damage the receiver, but it might increase the time the optocoupler needs to switch off, and thus foul up the timing of the UART signal (for really cheap and non-compliant receivers that are already on the edge of not working). Anyway, there's no reason to waste power.
The "A" is just a label used in the remark below:


  Gates "A" are IC or transistor.


When the MIDI specification was written, typical microcontrollers (e.g., 8051) were not able to sink 5 mA of current, so it was necessary to add some buffer. Nowadays, you would be able to use the output pin of most microcontrollers directly, or just use a single non-inverting 74LVC1G07 gate, but thirty years ago, the most easy way to construct a buffer would be to use two gates of a hex inverter. (Whether a Schmitt-trigger input is needed depends on the input signal of the inverter.)

What exact chip or buffer circuit you use does not really matter, as long as it is capable of 5 mA. (Many older MIDI devices indeed used transistors.)
A Thru circuit is just a normal output circuit connected to a normal input circuit.
The DSS306 are EMI filters. Some kind of filtering is likely to be necessary nowadays; this is one of the reasons that the MIDI schematic you mentioned is outdated and was replaced with the Electrical Specification Update (2014).

If you are not going to sell the device, or if you are a cheap bastard, don't bother about EMI. Otherwise, use something that filters out high frequencies (MIDI is slow, so you don't need more bandwidth than a few MHz). In practice, many devices use ferrite beads, but RC filters or even common-mode chokes are also used sometimes.
Transistors can handle larger currents and voltages than a logic gate, but that does not matter for MIDI.

A transistor is just cheaper …
… but if you're using a hex inverter, you might just as well use all of its gates.