If I'm reaching the upper Speed target, the a/p tells me to DECELERATE which I do using speedbrakes. You say it does not allow it... How? It does not have other options than to decrease descent rate by pulling the nose up, correct? Will it even do that if I do not use speedbrakes although it is telling me to DECELERATE?
DECELERATE will show up only if you pass over the TOD still at level but will dissapear as soon as you start the descent.
If by maintaining the upper limit of the speed range the airplane is still not able to get back on path, it may suggest you to use some speed brakes. Using the speed brakes at that time is not to decelerate, but to increase the rate of descent.
Still, the usual method to get back on path would be to select a higher speed, making sure to maintain IDLE THR by selecting OPEN DES.