Yep, it’s a bit of a “Catch 22” situation there … if you select them all, it… groups… them.
However, it will not group together notes that aren’t consecutive, so what I actually did, in that first bar, was select just the 8th-notes on beats 1 and 3, then the Tremolo operation gave the result that you see there. Over a long passage, that would indeed be only moderately less tedious than doing each one individually (i.e. selecting every other paired group of 8th-notes).
I haven’t yet tried it, but I suspect the desired result could be obtained in two steps via the Logical Editor (the first, selecting odd-numbered beats… then tremolo function, then a 2nd time, selecting even-numbered beats. Probably could make a couple of macros, to at least make that all just a two-click operation .
I have no idea what expression cancels a “sempre” (I’ll ask a colleague of mine, who just happens to be italian ).
What I usually do (although I’ve always used “sim” rather than “sempre”… but yours is correct ), is to write out in full the last group of notes that uses the “sim”, followed by first note/group of notes that no longer makes use of the “sim” (and maybe add another “sim” after that one).
Maybe it is “ordinario”?