Indents
Oh my God, wow... that's really weird. I'll have to double check all my editors. One of them must be deceiving me with how big it says a tab is. No biggie. I'll fix that ASAP.
Wire Speed
I typically use a Nano or a Nano Every for testing. And I can change I2C_SLOW to 50000, making the lowest speed 50kHz. But I'm not sure what you mean by "It could be a very high or very low value". As it's written, the value is a constant, or at least should be.
Writing Wire Data Delays
Okay, yeah, I see what you're saying. That makes extra sense when you put it like that. And it seems to be working with a short delay after Wire.endTransmission() only. So I've resolved that bit, and hopefully I haven't created too much of an issue for you yet. 
RS232 Delay
I'm all for trimming useless code and streamlining functionality, so I'm just going to eliminate those delays in the Serial functions. But I'll keep that in mind for the future in case there's some bug that emerges there at some point. But I doubt that it will.
I did not know that about the gap between data bytes being an extra long stop bit, no. But now I do. This is all very helpful information