^^
Make sure you get pipes that will hold the weight of the flowers. strong PVC pipes are great. They are hollow but sturdy.
Seems like your wont be able to attach the pipes to the leather sofas. And again, I cant emphasize enough that they need to be secure. You dont want them falling over!
If you do want to incorporate the sofas for extra seating, cover them with the fabric you've bought. It doesnt have to fit like a glove, just flow-y and colorful.
Thats actually quite simple. Get plates that are deep - That will hold water. Then for henna, candles and flowers get a few kinds, Some deep, almost bowl-like, some wider but still deep, some very small bowls and any other shape that you like. Fill the deep ones with water. Others, you will use for decoration. (put water in at the end - first set them up) Take a large plate or small tray. Put flowers on the outer rim. Put a deep bowl/cup/etc in the center. Fill with water and sprinkle rose petals. Now inside that bowl put the smallest cup/holder with a candle in it.
If you dont get any of this, let me know.
Use lots of different colors, ribbons, candles, sizes. You can replace the flowers on the outer rim with henna with candles embedded in it for a different effect. You can also, sprinkle glitter all over the flowers once you set them up.
PS: For tips on securing poles, you have to tell me where the mehndi is? Indoors, outdoors. Do you have a solid concrete ceiling or hardwood floors? Again, where is makhram?