Assuming that the practice of deferring convictions in exchange for military service is brought back into practice in the future (in Real Life, this has been specifically outlawed in the United States for several decades), it seems unlikely that someone would be sentenced to become a commissioned military officer and a Space Fighter pilot, as opposed to some kind of enlisted footsoldier or laborer. It's possible that the judge was manipulating the law to get Cooper Hawkes into a far better situation than the one he originated in (homeless and nearly murdered in an alleyway, only to be arrested by the cops afterwards).
The Chigs decision to seek a treaty with Earth after discovering about the invasion of their homeworld is a monumental operational security blunder on the part of the Wild Cards. However, the choice by the Chigs to negotiate rather than try to take military advantage of it is a good idea. This entire business is about defending themselves and their breeding worlds, which humans now know about. They also know individual humans are interested in protecting them versus seeking genocide. Before meeting the Wild Cards, the Chigs weren't aware humans weren't Scary Dogmatic Aliens.
Expensively trained pilots do not get used as footsoldiers. The makers were obviously aware of this as there's one scene where McQueen chews them out for complaining about taking on this role, saying it's their job to do whatever task they're given, and citing the old Marine Corps chestnut "Every marine is first and foremost a rifleman".
How bad must the casualties be if the Marines are using officers who are trained for flying space fighters for ground combat that any enlisted grunt can and should be doing in their place?
It's stated that for the first half of the series, the war is going very badly for Earth forces, hence the above. By the second half, things are going much better thanks in no small part to the 58th, who have become somewhat legendary, so it's possible they continued to be given ground combat missions both for their proven combat effectiveness and the high impact on overall morale.
The war hasn't really begun in earnest by the time the Wild Cards receive their training. Bizarre as it may seem, Marine starfighter pilots are trained to also do infantry duty. What sort of theater of war requirements or ideology behind that decision is left unexplained but Word of God is they just wanted to tell more kinds of war stories.
You can justify a lot with a Space War as getting into space and to other planets is the primary issue for every soldier. The costs for getting a soldier to a planet or enemy spacecraft are probably comparable for training a soldier to fly both a spacecraft and building one with greater operational freedom.