It depends on the software. If you have the version with the latched descent then the path takes priority over the speed. All altitude constraints will be respected. If you have the version that does not latch the profile then selecting a speed may result in the aircraft flying above the profile in order to maintain the speed. This can result in at-or-below constraints not being respected.
For your question 2, the same applies but you need to estimate when to slow down. With the profile latched the profile will be flown assuming a deceleration and so it should be apparent by it levelling at the constraint or shallowing the descent that it is expecting the deceleration to commence. You also have the magenta spots that indicate deceleration points but I can't remember off the top of my head if they remain visible in selected speed.