Can training pants replace diapers?

No, these are not diapers. There is a little more padding for absorbency but they can't hold a lot urine like a diaper. Training underwear is softer than diaper. A great in-between when transitioning from pullups.

Are these training pants be machine washable?

Yes, machine washable, but hand wash is of highly recommended which prolong the life of underwear.

Are they leak proof?

There is a good amount of absorption. But not leak proof. It does a good job for small accidents. Bigger accidents, it will eventually soak through.

Is this reccomended for nighttime potty training? Would this leak?

No, this will not work for night time potty training, it's not pull-ups and don't soak up all night's urine, it does leak, but relatively less than a regular underwear as our training pants have a thick padding.

Needed to use a special liner with these?

For a big accident you would need backup but for little accidents it absorbs pretty well.