Define the term - agenda amendments
The solution to the above lies in the rules of policy or agenda amendments and the real weapon in the hands of the voter is their ability to vote strategically, to get the policy agenda amended. In coalition formation, with a view to the "minimum necessary to win" as stated above, and given the policy positions of parties only rank order of the voting makers in determining the coalition.
In case all the parties fail and no party gets a majority, the first and third, ranked parties form the government. Note that the "third" may be a single party or a group of social parties or independent winners taking part in coalition.
Then policy positions are symmetrically distributed around the median voter's preference position; with one party adapting the median position. This party received the fewest votes, but is this assured of being a member of the coalition government.