And I welcome you to Shin’s deteriorating mental health. You can probably guess the factors for that. The happenings of the events to come in the upcoming episodes too, probably.
It’s not so much as forcing, but more logical to send the 86. They’re the best and most experience, so there’s a higher chance of them succeeding in the mission, even if it is suicidal. Also understandable why some would hate them/fear them, as they had a chance to live a normal life but chose to go back to the battlefield because of ‘pride’. A pretty toxic trait of theirs, unfortunately, as they despise those that pity them.
And whilst again it is a suicide mission, they’re not just being sent to die. They’ve got the Federacy of Giad backing them up, as well as two other countries, so the odds are already better compared to their chances in the Republic.
And with Shin and the ‘vision’ he had, it’s more in his head, reminiscing of what others have said in the past, from good to bad. Such as when he ‘heard’ Lena’s voice, it points to his right ear. It’s to represent his train of thought at the time. As for the second letter Shin received if you didn't fully see it, was a picture of a headless reaper: Shin.
Lastly, with the ‘.5’ episodes, they’re just special episodes for when production was going pretty awry and were having two-week breaks, so you can skip them. You can blame Anifex for that, and they're the ones that wanted to release it in 2020.