Foam does not reduce titanium sparks. Modern aircraft have titanium in them so it doesn't help anymore... It does impede rescue efforts and make a mess of an evacuation however. You can lose people under it etc...
As to downwards motion.
The center of mass of the aircraft in the forward vector is somewhere around the main cabin floor. And rearward vector below that point (like the engines) will cause a net pitch down moment while the whole assembly is in motion.
However, that is negligbable.
I suspect it had more to do with steering issues should one engine have fodded out and the other contineud to produce thrust, or the reversers open assymentrically.
Also deployment of spoilers produces and abrupt pitching moment, not something you are looking for when trying to be smooth with the nosewheel.
Cheers
Wino