Sellers; do they want to SELL or not? If they are not committed, they shouldn't have gone this far.
Buyers; they are acting in good faith, they should have their offer accepted.
Now the reality; I have yet to see an offer that was 100% clean, there is always something outstanding that would allow the Sellers a loophole. If they want one.
Remember that Contracts are for honest people with good intentions.
No, they don't have to. Sellers don't have to sell their house if they don't want to. Hopefully if they have it listed for sale and the pricing is correct they really want to sell it. The best way to get your offer accepted it to make your offer as 'clean' as possible - don't ask for closing costs, repairs, etc. Also, have your Realtor do a Comparative Market Analysis (CMA) on the property using SOLD comps within a 1 mile radius of the property (the closer, the better) that have SOLD within the last 3-6 months. This will give you current market value and this is what you should base your offer on - not on list price which may be too high or too low.