IT FINALLY WORKED!
I took one kinda sorta short cut and it really turned out well. From my previous errors, I knew the mayo would break if I over mixed as I added the oil. In the past, my problem was either I drizzled too slowly or too quickly. This time I used a small food processor and added oil in 1/3 cupfuls and pulsed 6 times after each addition. After the last 1/3, I ran the processor for... maybe 5 seconds? until I saw the mayo thicken and I stopped.
I threw some in tuna with my own spice blend of salt, pepper, onion powder, garlic powder and tumeric (hence, it's very yellow). I ate this alongside some halved cherry tomatoes.
It came out sooo yummy that it's toddler-approved!!