I am so sorry to hear about your diagnosis
There is a hemangiosarcoma link in this cancer thread that might be helpful:
There is an herb that is supposed to help stop bleeding that some have used, the antibiotic doxycycline has been used to prevent the formation of new blood vessels to the tumor.
Is the vet sure that there isn't a tick disease - the blood profile and enlarged spleen can be found in both. When my boy had low plat, lower red blood cell count and an enlarged spleen, the ultrasound showed no tumor, just a "generous spleen". Of course, if this is related to this fatty tumor you mention, then it wouldn't be tick related.