I tried to answer this for your previous question but deleted my answer as I felt it was poor and may have overstated the case. I'll try to keep my answer limited as a result:

I'll start with your second question first, which in my opinion goes a long way towards answering your first question.

There are currently two dams across the Nile that provide flood control and electricity generation. These are the Aswan Low Dam (constructed 1899-1902) and the Aswan High Dam (1960-1970.) The Aswan Low Dam was the first successful permanent dam across the Nile.

For their time, each of these dams was among the largest and most sophisticated engineering projects ever attempted. The Aswan Low Dam at the time of its construction was the largest masonry dams in the world. It's a really imposing bit of fin-de-siecle engineering: https://paperarch.files.wordpress.com/2012/08/low-aswan-dam-bw.jpg

Despite that, even the low dam wasn't quite sufficient to control the Nile flooding-- the dam had to be raised twice and was nearly over-flowed in 1946. It also wasn't designed as a hydro-electric dam.

As a result, Nasser pushed for the building of the Aswan High Dam. Aside from some really interesting cold-war geopolitics involved in that, from an engineering perspective it also mean the creation of Lake Nasser, which is one of the largest man-made lakes in the world.

All of that is a preface to saying that while I don't know the specifics of ibn al-Haytham's attempts to dam the Nile, I don't think it would have been even close to possible using 10th/11th century engineering techniques.