I'm so sorry for your loss. I can't even imagine!
Here is what I know about marble and granite care. 1st - someone told me years ago - to NEVER use any cleaners or polishes that have a "petroleum" product base - as they are porous stones and the product will settle into the pores and turn yellow and eventually brown. :-(
I have granite counters in our bathrooms - black in one, and a dark black, gold,green variegated one in the other. They both water spot like crazy! I found a product at Publix - that says " Marble & Granite cleaner & Polish" - it seems to take the water spots right off, and puts a "slick shine" on that pretty much repels the water after that. I spray it on, wipe it around with a paper towel. At this point it looks like a smeary mess, and then I rub it out with a micro fiber dusting / polishing cloth and it shines and is perfect looking after that. Depending on how messy they get - I usually only do that once a month or so - and just give it a wipe with vinegar and water in between. I would keep trying to reach someone by phone at the cemetery or at a vendor of such products to ask their opinion as well. Good luck!