"This gait is used...to steepen descent and at the same time...not pick up excess speed"
...the same effect can be achieved with forward slip?
Yes, you can steepen your descent angle at the same speed with a forward slip.
"This gait" is an extreme example of "short field" landing technique, but first:
any maneuvering near stall speed close to the ...
It really depends on the aircraft. Some have plenty of rudder authority and sideslip very well, others are better at stall onset with still plenty of elevator and aileron authority.
Now first to the term "mushing glide": This happens at very low speed when the airplane flies far at the backside of its envelope. Flow on the inner wing is already partly ...