Re: Myrobalan Plum as Root Stock
This wont' answer all your questions, but I'll take a stab at some of them. I know I like getting responses even if they aren't comprehensive in their coverage of my inquiry.
I don't have personal experience with them, but Myrobalan plum are a common stone fruit rootstock. If yours is a seedling it may have somewhat different properties than the ones selected for clonal propagation but I'm sure it will make suitable rootstock for grafting.
This site sells Methley on a Myrobalan clonal selection: http://www.sanhedrinnursery.com/detail/173-methley
This site sells Moyer on a Myrobalan seedling: http://www.greenmantlenursery.com/fruit/plums.htm
I've had good results grafting dormant scions onto established trees using bark grafts and whip and tongue in April/May and believe bench grafting would work as well. I think many prefer budding stone fruit mid summer.
If you like the fruit, by all means, propagate it and let it grow and produce more of it for you.
If you are looking to share or trade, I wouldn't mind a sucker or two after reading your description.
I don't know how common the practice of grafting those types of trees would be at the time it would have been planted considering its age. On that note, is it in a location that suggests it was planted intentionally?
Pictures may help others give a more informed opinion with regards to whether it is in fact Myrolaban and perhaps whether it is grafted/budded.