As far I know yes, once you get to the quality that you want you can always keep it for relic
Yes that relic quality holds through ancient gear and beyond
Plz correct me if Im wrong, Yoz is tradable so if for those who have good RNG can sell their Yoz in the market, so why remove it?
Even more incentive to gamble.
A lot of people would buy the boxes hoping to get some legendary pieces in order to pretty much get better gold exchange rate than they would via usual royal->gold.
What do you mean by a quality upgrade? Each time I gamble when I hone I burn 20$ in gold with mats with 1% success rate with everything juiced up. At least Yoz is 10% chance every roll and I can buy any pieces i need from AH.