A subwoofer or two is almost always helpful with small monitors if deep bass and anything other than low level is desired. There is then the possibility of placing the main monitors where they perform best and the subwoofer where it/they perform best.
If there is no subwoofer a compromise has to be struck. Bass is a bit like overall frequency response that if it is wrong nothing else matters as much. If the whole bass is missing that is less problematic than if random parts of it are missing. You can easily listen to a table radio and not be horrified by the sound as it has no bass. But something that puts a one note boomy bass over everything is much more objectionable.
Headphones are also a valid companion to monitors that do not have full range capability to make sure that nothing untoward has occurred in the sub bass during a mix that couldn't be heard through your monitors.
Even though your room is not completely rectangular it is rectangular enough that the REW simulation should give good insight into speaker positioning and boundary interference. If you can find a position that reduces destructive interference and works practically then it is likely to be closer than moving randomly which is not going to be easy for you to do. Peaks are not much of problem as they can be EQ'd but big dips need to be avoided.
Your original position and treatment probably was not too far away from the best compromise you can get. I would imagine there is middle ground between no treatment and close wall placement and full treatment thickness forcing the speaker further away than optimal. 4 to 6" of absorbent is enough to be broadband enough and kill most early reflections in it's path.
As the room is quite long you might want to consider moving the speakers and desk into the middle away from the wall behind them. If you can get them to 2m from the wall the reflections will then be much less detrimental but you will lose some boundary reinforcement too.
For understanding envelopment and many other acoustic concepts read the work of David Griesinger formerly of Lexicon. His website has most of his papers and powerpoints available.
http://www.davidgriesinger.com/laaes2.pdf
For seeing the level of early reflections an ETC of the impulse in REW is helpful.