It would have technically worked to add him in there, but the show kind of intentionally avoided cameos because they're a distraction from the main story. The only already established characters in Andor are ones that were necessary to drive the plot forward. Adding this cameo would have done nothing for the story of Andor and would have ultimately been a distraction that would have pulled focus away from all the other things happening in that scene. The scene was too critical to or distract from with a cameo, even a cameo that would would have fit with the already written script. Also the character would have been 10 years younger which might have pushed the boundaries of believable enough to break immersion, especially when there was no need to have that particular character there anyway. Andor did a great job of really just focusing on the story that it was trying to tell and this would have gone against that.

The marriage really wasn't a choice that Leida was making. It was chosen for her. The decision for her to marry at this age and the choice of who she would be marrying were made by her mom, and was done out of political necessity more than tradition. She wasn't happy with who she was marrying, but felt like she had to go through with it anyway. Mon Mothma gave her the chance at the last second to back out, but she likely didn't feel like she really had the freedom to do so. Suggesting that she had the choice ended up almost being crueler than just telling her she had to go through with it, making it seem like she had a choice when they both know she really didn't. She had to go through with it to save her family and live up to social expectations.

I think that's what made it all the more disturbing. It wasn't Leida choosing this fate. It was Mon sacrificing her daughter to this fate to save her own neck. This is reinforced when Mon likewise sacrifices her childhood friend in the same episode, also to save her own neck and the rebellion.

The Battle of Endor happened in 4 ABY, and the final season of Andor said it took place at 1 BBY, so I'd say 5 years, plus probably 1 additional year before the empire officially cedes power and its prisoners are released, so 6 years? Physically she'd be able to handle the labor for that long, but mentally that would be a long time for her to last.

As far as the kinderblock, the Empire officially began 18 years before the final episode of Andor, but maybe it was a Republic kinderblock, that then got rebranded as an Imperial kinderblock, and she just sort of refers to the entire time as the Imperial kinderblock. I suspect there's a lot of that in the Imperial era where institutions that existed before just get rebranded and continue on like they were always the Imperial institution, because in many ways they were. The Empire was really just a different name for the Republic with a Chancellor that no longer has to worry about reelection. The Republic Senate became the Imperial Senate, the Republic Navy became the Imperial Navy, and the Republic kinderblocks became Imperial kinderblocks.

I kind of had a similar thought. She and Phasma have a very similar personality. Phasma's unflinching willingness to brainwash people, interest in investgating key individuals and events, and desire for an orderly world all fit. The Force Awakens happens 35 years after the end of Andor, so she'd be kind of old by that point, though. Not super old, but she'd be retirement age by then.

They address the terrible aim a little bit in Bad Batch where the new troopers get upgraded armor that also makes it hard to see. I think they reference that in Rebels, too, when Rex or one of his old clone buddies remarks that it's no wonder storm troopers can't hit anything with those helmets. I think of them mkre as the riot police. Heavily armored against melee attacks, able to create a lot of carnage, and scare thebdaylights out of protesters, but maybe not properly equipped to be chasing a single agitator in the streets.