Can a gastric sleeve directly help with herniated disks / pinched nerve and knee problems (meniscus)?

Not directly. No, it will not help directly, but if with diet and exercise it allows you to lose weight, so of the symptoms associated with those other conditions may improve or even go away. Excess weight aggravates many conditions, including the ones you mention.
Not directly . Indirectly, the weight loss helps by taking pressure off your joints. Obesity also leads to inflammatory hormones, which are reduced after weight loss.