I admit I think the first part is really a question for Captain Everett but I'II give it a shot for now. Wickham is going into the regular army. I think this is above the militia. I am reasonably sure an Ensign is one of the lowest ranks, below Lieutenant. As we hear the 'commision' was purchased, it may a soldier without formal training assigned an officer's rank. Regarding Mr Gardiner's letter,(unless it's just a term of phrase I'm taking too literally) I'm inclined to think 'he promises fairly' means as Wickham is getting a fair start i.e new commission, some money, new place and different people- he has every reason to behave more prudently and responsibly in future.Perhaps Wickham has given Mr Gardiner his assurance. IMO, Mr Gardiner can only hope Wickham intends to commit to his word. What chance Wickham will fulfill his new opportunity? ;)