for point 5th, If you close the card in the 11th month, you will not be charged with the fees. And closing the oldest card may decrease some points in credit score, but I believe it won't be much, since you have the same age card as well.
You never know, decreasing the number of debt cards also has a possibility of increasing the credit points.
For point 4th, I think Millenia gives 5% cashback on Amazon and Flipkart spends, so you can use the card to buy the Amazon gift voucher and Flipkart Gift voucher, using the Amazon and Flipkart website itself and sell it on this forum trading section in some discount.
The normal rate of trading section for AGV is 2-3% and FK GV is 4-5%.
and if you buy Flipkart GV using Flipkart via HDFC Smartbuy, I think the cashback will be even go as high upto 6-7%.
For 2nd point, I don't think you can do anything, just sit back and relax enjoying the card, it's one of the best cards for online shopping on amazon/Flipkart.
Who knows you might get another upgrade/LTF offer within a year.
For 1st point, it looks true, my friend also got the same offer as you. He had moneyback LTF and got millennia FYF, he was very happy to get it, as the cashback rate is very attractive if you do a lot of shopping.