You can have a go at the ridged shaft if you like but bare in mind there is a reason for both quality conversions on the market having flex couplings. The test will be bearing and/or shaft life.
As far as dust control for the 10SI there are two options for mounting screens on the back of the alternator. One is the marine application which uses a combined flame arrester/screen consisting of three layers of fine mesh built in to a cover housing. The other is referred to as a "chaff" screen which is similar to the marine unit but one layer of screening and a lighter cover housing.
Bearings in the 10SI are the needle bearing prelubed type on the back end and prelubed enclosed ball bearing on the front.......adequate but not what I would call beefy.
Got the "Supper" call, will add more later