Doesn't apply here, say for example i have a piece of steel with length 100mm and it stretches 10mm, is mm/mm the strain would be 0.1 mm/mm, in meters it would be 0.1m/m
Really strain is dimensionless but occasionally people add units
Because practicality. Strain generally occurs across mm scales at most for most traditional tensile tests and relevant materials. Normally it's actually much less than mm. Occasionally you see micrometers/micrometers.
Because excel doesn't have built in unit handling so when you enter in readings from the strain gauge you'll probably enter them in what's being reported.
You can write the units of strain however you like, I often say ul for unitless.