The B&Ws sound profile will be closer to you your Infinity Primus's. It will be forward, even a little bright. The Monitor Audio's will be more neutral. IMO, there is not comparison. The MAs are infinitely better but that is a personal preference. Obviously a lot of people like the more forward sound of B&W. If you really like the Primus's you may find that the MA 100s will seem a little laid back in comparison.

20cm will be a challenge with any rear ported speaker. You will want to insert a port bung. You might also want to check out the Focal Chorus line. They are front ported.

In the end, even front ported speakers may not do well if they are too close to the back wall.

I have heard the Kef Q300 and was not a huge fan but that is my personal taste and is worth precisely what you paid for it.

I think that if you the Infinitys you will like the Kef's as they tend to be a bit forward. They will have a relatively long break in period so let them burn in for a couple weeks before you form an opinion. I say this because another user out here replaced his Primus's with the Kef Q line and loves them.