Are Lucky Pokemon guaranteed to have better IVs

The only guarantee is the IV floor of 12/12/12. It’s not guaranteed to automatically be better or worse than it previously was

No. With 96 Breloom after trading you are very likely to get a worse one.

Do check if you have any July/August mons with legacy moves, like SC Gengar, RT Omanster, Psybeam Chansey or DC Dragonite. They could be very valuable and you can ask for good mons in return.

Guaranteed 12/12/12 minimum IVs. That's better than egg hatches. But yes, if you trade a 96% pokemon it can go down and most likely it will.