duct system affecting delta?

I live in Tulsa and its been about 100F here lately. In 2019 I had a carrier 24acb7 installed. It hasn't ever performed like I had hoped. In the last two weeks, the company that installed has replaced the evap coil, this was because TXV was bad, and then replaced the entire condenser unit because the tech that changed the evap said the coil wasn't getting cold enough. The day they replaced the outside unit the delta was about 10 at best in 100F. We have high humidity in the house all the time 64or higher. Now they are saying the duct system is undersized. I made them add a return during the initial install. They installed a 12" flex on a 14x24 vent. The other return I believe is a 14" duct that transitions to a 16". Does that even do anything, other than decrease Velo? Their solution is to change supply from 14 to a 16 and put a box in and do "homeruns". Currently, there is just a bunch of wye now. I have 10 registers (7x7).

Q1) Going from a 14 to a 16 duct on return. What does this accomplish?

Q2) Does their purposed solution seem like it would fix the issue of poor Delta and high humidity?

Q3) if q2is no, then where is the issue?

Q4) If the supply is upgraded to a 16" what's the max size of the box, and how many taps can be used? I assuming the following because I asked for specifics and haven't gotten any. If they use 5 taps and use 10" that wye's to (2) size inch flex duct will that work?


Thank you

source https://hvac-talk.com/vbb/threads/2240160-duct-system-affecting-delta?goto=newpost