1. 3D Touch on iPhone lives for 4 years starting with iPhone 6 and ended with iPhone 8/XS. Apple was too stubborn to admit that TouchBar was a failure similar to the Butterfly Mechanism Keybord, plus the removal of MagSafe, HDMI and SD Card slot. Apple has had many short lived products such as the G4 Cube as an example as well. If Apple can get OLED at a significant discount it will introduce that either in the Pro line or may jump directly to the consumer line or maybe include it across the board however one having ProMotion and higher brightness while the other not so. Apple has done this with the iPhone 13/Pro line.
2. Just because there is no budget iPhone model with OLED today that means Apple will never offer it, great assessment considering history indicates as you said OLED was on the top level iPhones and have made its way to the consumer level models starting with iPhone 12. miniLED not available on the iPad Pro 11” is an odd omission and maybe due to availability or a test bed.
3.
MicroLED is working, what you are referring to is getting it produced in large quantities with QC and various sizes. Give it 3-5 years, I suspect it may show up on an AW first then iPhone/iPad and Mac laptop and display. Apple usually is slower at moving onto new things such as OLED but that is due to many reasons.