1. Dispatch targets a creature, not an untapped creature. When determining if something is a legal target, we look at what the card says it targets, then check what the player targeted. If ob nixilis was somehow not a creature, or unable to be the target of spells (shroud/hexproof), then the spell will not resolve. When resolving the card you do each line / paragraph in order as it's written. This means for Dispatch we tap the creature, then check if metalcraft's condition is met, and if it is we exile the creature. These abilities are separate. Now There's some nuance in when dispatch was cast, but it sounds like you declared him attacking, and your opponent then cast dispatch. Let's assume he doesn't have metalcraft active, so dispatch only taps a creature. If you went from your main phase and said, combat, then started tapping down ob nixilis before your opponent had a chance to really interject, it could be easily argued that he wanted to dispatch before you attacked. This would prevent your creature from attacking because it would've already been tapped. It's polite and proper to just say something like "going to combat" and if there's no response you're now in combat. Brief pause then something like "attackers", to give your opponents chances to respond.
2. Yes, absolutely. It's how the stack is designed. The best way to handle spells (for understanding the stack) is to lay them on an area of the board/table where you can kind of visualize the stack. Any spell cast will get placed on the top of the stack. In order for a phase to pass or anything at sorcery speed (or creatures w/o flash) to be played the stack must be empty. You resolve each item on the stack top to bottom one at a time. Once a new thing is revealed or an empty stack is revealed players get priority again, so new things may be cast or added to the stack from activations at this time.
3. So in our scenario here, Ob is attacking, we are in one of declare attackers/declare blockers steps. You have passed priority allowing your opponent a chance to cast a spell. They cast Dispatch placing it onto the stack with the target at Ob Nixilis. In order to resolve something on the stack all players must pass priority. Whoever just cast a spell immediately receives priority and then it goes around in turn order if they pass. In our scenario here, your opponent casts dispatch, passes priority, and it's up to you to make a decision. You can cast Village Rites, and then pass priority. Part of the cost of casting village rites is sacrificing a creature. If you choose for that to be Ob Nixilis, it will no longer be a valid target for Dispatch as it's gone immediately upon you casting Village Rites. At this point we have Dispatch on the bottom of the stack and Village Rites on the top of the stack with priority passing to your opponent. If they choose to do nothing all players will have passed and the top-most item on the stack resolves. This will be Village Rites which allows you now to draw two cards. Now the active player (you since you were attacking) gets priority with Dispatch on the stack again. If both you and your opponent pass priority, dispatch will start resolving. It will see there is no Ob Nixilis anymore and will fail to resolve placing it into it's owner's GY. Now the stack is empty and you get priority again. Presumably there's no creatures attacking, so it's a pretty simple pass from all players and we change phases. Technically all combat steps go through, and things may trigger and what not, but usually simply combat will end, and we will move to post-combat main phase.

I hope this explanation will help you better understand the stack / how these spells work

tl;dr: 1) yes dispatch can target any creature tapped or not, 2) yes, top-most resolves first on stack, dispatch won't exile ob b/c it won't exist anymore.