Jeff;
Are there formulae or tables to compute the time needed to reach Mach 0.85 and Mach 0.90 starting from FL350, Mach 0.80, according to several descent angles (1°,2°,3°...) and different thrust levels (N1=0.85, 0.90, 0.95, Max) ? is it possible to compute the same thing but with a banking angle ?
No. Such tables would serve no useful purpose or solve no problem which carries risk. No airline pilot has a right to expect that such tables would be used or useful, mainly because most seasoned pilots know pretty well what it would take.
To regain speed at high altitude it takes a very long time in level flight especially if the air is warm, (ISA + 15 and warmer). However, speed can increase very rapidly even with a 0-degree pitch or perhaps 1, maybe 2 degrees at most, depending upon power setting and what the air is doing around you. Most transports cruise at 2deg NU or so - some slightly higher, others a bit lower and it is of course, dependant upon IAS. A five degree nose-down pitch is a very serious pitch down and 10 degrees could be defined as a loss of control without speed/Mach limit.
It's very simple and explained very well in Davies. The L/D curve for a jet transport when Davies wrote back then and still today, is very shallow. The airplane will slide along the curve left and right, very easily where as for propeller-driven aircraft the curve is much more "U" shaped mainly due to the thrust and drag offered by the large disc(s) described by the props.