I would analyze it as follows:
is indeed E minor. Since you go from D back to Em, it has a "modal" feel - Aeolian or Dorian. In a diatonic E minor scale, I'd expect to see a d# somewhere. (Not saying that you should, it sounds fine)
Still in e minor. As you repeat it more often, it might start to sound like G major (the relative major of E minor)
Without any information about the rhythm you're playing it can be difficult to draw a conclusion, but to me this sounds you're still in Em; in fact, C can be seen a a substitution chord for Em (third lower) and Bm as a substitution of D (also, a third lower).
(Not sure of it fits your song but if you have a longish pattern repeating Em D, you could occasionally substitute Em with C and D with Bm just for variety. If you don't overdo that, it may start to sound as an "announcement" of the C Bm progression in this bridge)
This is where it becomes interesting. The progression of a major chord followed by a major chord a semi-tone below it often sounds as a VI-V progression in a (melodic) minor scale. The scale that would in this case be Dm; For this reason I would probably analyze it as:
Rather than A# A.
Now you proceed with:
Note that this is exactly like Em D, but a fourth lower (fifth higher). Now, I just argued that the Bb A progression suggested Dm. But the A could just as easily progress to D major. And interestingly, Bm is a substitution chord (third lower) of D.
At this point I will argue that the progression from A to Bm is actually an interrupted or deceptive cadence (http://en.wikipedia.org/wiki/Cadence_(music)#Interrupted_.28deceptive.29_cadence). That is, the A raises expectation to go to D major, but instead, it progresses to the relative minor of D major, which is to say Bm.
Going back to Em is simple - Tim already pointed out that you can simply use a B7. Since you're coming from A, and A B7 is in the melodic minor scale of E minor, the move back to E minor will sound very natural, and doesn't need any extra chords or preparation.
That said, you could spice it up a little to again use substitution chords, this time for Bm and A. These would be G and Fm respectively. So you could do
Bm A | Bm Fm
And then restate your C G chords that got you there:
If you then let the C follow up by a B7, you get a alot of tension that make the return to Em feel inevitable. (This C B is same thing as what I mentioned before about Bb A - the C B will sound as VI V progression, in this case in the scale of Em)
G C | B7 B7 | Em