Is it possible to set the engraving defaults so that bar numbers align above and to the left of the staff?
I can move each one in Engrave mode, and looking at the properties panel under Time Signatures (?huh?) it would seem that I like the placement as
Bar number X = -1 1/4 & Bar number Y = -1/2
How can I make this the default placement for all bar numbers?
In Library > Paragraph Styles, setting a negative value for First line indent shifts ALL bar numbers to the left, not just those at the start of a system. If you want bar numbers only at the start of a system, this might be a possible solution. If you do want a few bar numbers on bars which are not the first bar in a system, you would need to move those manually as you described in your earlier post. You would need to set the First line indent value such that the majority align how you want them and then adjust those which are not quite there. You can select more than one in Write mode and then adjust them all at the same time in Engrave mode.
I have been using first line (out)dent as default for a couple of years, and it is not ideal. In addition to the reasons you mentioned, single digits are too far left.
My ideal bar number placement is centered over the system barline, just like other barlines. With default spacing gaps, even 3-digit numbers would not collide with a treble clef.