A1000:
I found some wiring diagrams that seem clarify matters for me. This site has wiring diagrams for DB9 connectors, DB25 connectors, and a DEC MMJ connector.
All sorts of stuff there, and if it helps with the concepts, all the better. Wikipedia is pretty comprehensive of course.
A1000:
Is a DEC MMJ the type of connector used on a USB to TTL cable such as this one at Amazon?
Nothing to do with DEC MMJs. The simple pin and sleeve connectors seem to be referred to as "Dupont" connectors, the sleeves come in various geometries such as singles (as in your reference), two, three four, five, six and so on in line, where six would fit that particular device, then two by two, two by three and so on. On eBay and such, ribbon cable fitted with the (single) connector pins - female or male - is available which can be divided into strips of say, six wires and the (single) sleeves supplied carefully removed and replaced with multiple pin sleeves, re-arranging the sequence of the wires if necessary. That is what I have been doing to make connector leads for particular purposes with the Arduino modules available.
A1000:
If I wanted to create a one way connection using two of the above USB to TTL modules, would I connect the wires as follows?
USB Module plugged into Computer1 USB Module plugged into Computer1
TX- Transmit Data <-------> RX + Receive Data
TX+ Transmit Data Ground <-------> RX – Receive Data Ground
Exactly. And you just might wish to implement a hardware handshake connection back from the receiving one to the transmitting (if you are content that this cannot be hacked, as previously considered), in which case it is notable that (as far as I can determine from the illustration) that the particular item (and they are available much more cheaply, for what it is worth) has a design fault in the process of copying from an original design used with Arduino and other MCUs. You can however pick up the necessary handshake lines by fitting extra pins to the board for RTS and CTS.
A1000:
Could I also do it this way, plugging directly into the DB9 port on Computer2?
USB Module plugged into Computer1 DB9 port on Computer2
TX- Transmit Data <-------> Receive Data Pin 2
TX+ Transmit Data Ground <-------> Signal Ground Pin 5
No, because RS232 uses different voltages and polarities to the USB to TTL adaptor. You could however do that using one of the readily available USB to RS-232 adaptors with the DE9 ("DB9" is actually incorrect) connector and the appropriate wiring between the DE9 connectors.
{There is some understandable confusion here as the USB to TTL adaptors are commonly misleadingly advertised against the term "RS-232".}
A1000:
Paul-- In researching this project, I came across some really high end data diodes. If one-way data transfer using a serial connection is so effective (not to mention low cost), what are high end data diode companies really offering with one-way transfer diodes that cost thousands of dollars a piece?
Without knowing what these expensive products actually do, I cannot say exactly, but I can say that there is just no way that information could leak back across the two wire TTL or RS232 link. 
A1000:
Taking this further, could a one-way serial connection conceivably be breachable? I must admit that I know next to nothing in this field, and perhaps therefore my scepticism could in some small measure be justified, but if you could explain why a one-way serial connection would be adequate, I would be most receptive the idea.
The one-way link, particularly without handshaking - is going to be unbreachable. Except of course, by "side channel" attacks (something also discussed by Steve Gibson on many occasions) where some other form of communication or information is collected, such as detecting radio frequency radiation, sound, lights flashing or fluctuations in the power supply. But that is a "whole different ball game". Other than that, and as with the consideration of handshaking, transmission of data in the opposite direction is only possible if both machines are complicit; it would be necessary for the machine sending the "handshake" to encode information on that line differently to the normal on-and-off handshake, and for the machine receiving it to interpret this abnormal protocol in a meaningful fashion.
A1000:
Putting an Arduino in the middle introduces a some complexity, but as an novice, I view it as putting a comparatively simple device between two complex devices. My reasoning is that if the simple device were altered (ie hacked) it would be easier to recognise the breach because it is so simple to begin with.
I don't think so - it certainly would be (much) more difficult to "hack" - change the firmware - of the Arduino (except that if it contains the bootloader, then it can easily be re-programmed from the USB side), but there is no means it would then have to alert you of the fact.
