Because your marrow is hypocellular, the doctors are probably trying to figure out whether the "marrow factory" is broken or whether something is attacking your marrow/cells (autoimmune).
Since your counts are holding, and the cells that are being produced are healthy, it seems like something could be destroying them. (I'm not an expert though. This is just my opinion.)
I thought the comment about an accessory spleen was interesting. I did a little google research, and it does say that people without spleens have Howell Jolly bodies in their peripheral blood. Since you don't, perhaps you do have an accessory spleen and perhaps it is inflamed and behaving badly (destroying blood cells), although it is odd that your platelets are still good. It may be interesting to get an ultrasound to learn more.
Also, there are some anecdotal reports linking Singulair to Aplastic Anemia (again from google research). So you may want to discuss this with your doctor. Also you may consider getting tested for Vitamin A toxicity, which can happen from retinol A.
I agree with Marlene that you should rule out the hereditary causes despite being "older than typical". Are you still waiting for the cytogenetic reports? Those could be very telling.
53 yo female, dx 9/08, AA/hypo-MDS, subclinical PNH, ATG/CsA 12/08, partial response. small trisomy 6 clone, low-dose cyclosporine dependent