Option 2 for the tempo indication. It's the beat that matters.

You can go to the parallel minor for the Trio if you like. It's not required. (Is it even THAT common?) If there is a 'rule' for a Scherzo and Trio it's that, as a descendent of the classical Minuet and Trio, it's in a fast 3 time. You're breaking that rule! (And that's just fine.)

(If you DID want the two-8ths = three-8ths metric modulation, MuseScore will do it with a bit of editing and the Special Characters menu. The triplet number is trickier - not saying MuseScore can't manage it, but I haven't worked out how yet! You wouldn't need it anyway. 8th groups in 6/8 aren't triplets. Otherwise music in 6/8 would be knee-deep in triplet numbers.)

Option 2 for the tempo indication. It's the beat that matters. You're saying 'quqrter beat becomes dotted-quarter beat'. Probably what the players would assume, but good to spell it out.

No, I wouldn't inflict 6 flats on a string quartet! In this age of free-ranging and even atonal music they are quite accustomed to playing any notes in any order at all. But if, as I suspect, you're writing in a tonal style, there seems little reason for E♭ minor.

If your ear hears a 'parallel minor' move, MUST you start in Eb major? Why not D major or E major? But there are plenty of other places to go. You can go to the parallel minor for the Trio if you like. It's not required. (Is it even THAT common?) If there is a 'rule' for a Scherzo and Trio it's that, as a descendent of the classical Minuet and Trio, it's in a fast 3 time. You're breaking that rule! (And that's just fine.)

(If you DID want the two-8ths = three-8ths metric modulation, MuseScore will do it with a bit of editing and the Special Characters menu. The triplet number is trickier - not saying MuseScore can't manage it, but I haven't worked out how yet! You wouldn't need it anyway. 8th groups in 6/8 aren't triplets. Otherwise music in 6/8 would be knee-deep in triplet numbers.)

Option 2 for the tempo indication. It's the beat that matters.

You can go to the parallel minor for the Trio if you like. It's not required. (Is it even THAT common?) If there IS a 'rule' for a Scherzo and Trio it's that, as a descendent of the classical Minuet and Trio, it's in a fast 3 time. You're breaking THAT rule! (And that's just fine.)

(If you DID want the two-8ths = three-8ths metric modulation, MuseScore will do it with a bit of editing and the Special Characters menu. You don't need the triplet number. 8th groups in 6/8 aren't triplets. Otherwise music in 6/8 would be knee-deep in triplet numbers!)

Option 2 for the tempo indication. It's the beat that matters.

You can go to the parallel minor for the Trio if you like. It's not required. (Is it even THAT common?) If there is a 'rule' for a Scherzo and Trio it's that, as a descendent of the classical Minuet and Trio, it's in a fast 3 time. You're breaking that rule! (And that's just fine.)

(If you DID want the two-8ths = three-8ths metric modulation, MuseScore will do it with a bit of editing and the Special Characters menu. The triplet number is trickier - not saying MuseScore can't manage it, but I haven't worked out how yet! You wouldn't need it anyway. 8th groups in 6/8 aren't triplets. Otherwise music in 6/8 would be knee-deep in triplet numbers.)