If one of the targeted lands isn't a legal target as Shifting Borders resolves (say, because it has left the battlefield or become untargetable), the exchange doesn't occur.

You reveal all cards you intend to splice at the same time. Each individual card can be spliced only once onto any one spell.

A card with a splice ability can't be spliced onto itself because the spell is on the stack (and not in your hand) when you reveal the cards you want to splice onto it.

You choose all targets for the spell after revealing cards you want to splice, including any targets required by the text of any of those cards. You may choose a different target for each instance of the word "target" on the resulting spell.

If all of the spell's targets are illegal when the spell tries to resolve, it will be countered and none of its effects will happen.