I'm going to suggest an alternative approach here that should get you what you want and still leave the data in a conventional format.
As I pointed out in my comments on your other question, if you change the actual one-way attribute you will cause problems using network analysis software and confuse others working with the data because you won't be following the one-way and To/From convention generally expected (line direction, not cardinal direction). As mentioned in my comment above, one solution would be to flip the line and modify the one-way attribute from a binary yes/no to a different system as discussed here and below. But if all you're after is an attribute that has the cardinal direction (or a binary choice based on a common direction) of the one-way direction of travel, there's another way.
First, create a new field
OnewayCard or similar. This will hold your system's DoT value. This can be T or F (confusing in my opinion), or N and S per your other question above, or you can break it down further to N NE E S SE S SW W NW, or spell them out - it's up to you.
Next, rather than determining this value by comparing lat/lon of the nodes as you identify above and are trying to do in this question, let's just look at the cardinal direction of the line by considering its angle. I don't know the exact way to do this - I think there's a function you can call to get it, and I saw this other question that actually has some code that will compute it. The goal is to get the azimuth, or full-circle bearing, of the line (0 and 360 degrees being north, 90 degrees east, 180 south, 270 west, etc).
Now we'll use an if statement - for simplicity sake I will follow your two variable example from above and the other question, but you could add more cases. If azimuth is between 0 and 90 or 270 and 360 then
OnewayCard will be N (or north, or F). If azimuth is between 90 and 270 then
OnewayCard will be S (or south or T). Of course you'd add more cases if you wanted the quadrants and/or true directions.
This method should work since your data just has yes/no for one-way and therefore the only possible direction of travel on a one-way is with the line. If your one-way variable allowed for other settings, such as the (0, 1, -1) system I mentioned here or just (To, From), you would need to add another level of if statement that would compare the line direction to the one-way restriction before assigning DoT. For example, a SW direction line with a -1 (against, or From) one-way value would actually have a NW DoT.