seems to me like a lot of work, and your complicating what is not so complicated

if you have oil problems just use a good oil sep and you will have your problem solved without any headaches, i suggest the Danfoss OUB1 the version for brazing

autocascading the last stage will take the main advantage of using "normal" stages, that is, being able to hold load at close to 0 psi

also be carefull not to "over do" your high stages, a stage were the HX temp is too low for the refrigerant that is condensing will have very low pressures and that will make it not hold any load
each stage should be design with an objective and with knowned boundaries, refering to the refrigerants that will be used