Leanne,
The Psalm needs to be sung from the same lectern from which the readings and the Gospel are read.
The Alleluia can be sung from either the left or right side depending on whether you are looking at the altar or facing out from the altar. I'm not sure if you would call that an ambo. If there is no cantor, then the Priest or Deacon will simply read the Alleluia before he proceeds with reading the Gospel from the lectern or the pulpit.
Now, like anything else there are accommodations that are allowed. For example, if a parish is having an outdoor Mass and there is only one microphone with only one lectern available, it's considered a kind of temporary set up so all this stuff is within reason and isn't something you want to get all hung up on.
Certainly, if they're building a Church, the set up needs to be correct but reasonable exceptions can be made. You might have cantor who has difficulty walking or getting around so he or she stays at ambo. There are any number of acceptable reasons to make exceptions for this rubric.
Most of the time, if something is setup incorrectly and if there is no acceptable excuse, it's simply because they don't know any better — even though they should.
John |