'Most' is a quantifier that can be used when diagramming statements. 'Most' statements can be reversed using 'some.'
Example:
Most squirrels are grey.
S - most - G G - some - S
The word 'probably' is used to make a generalization or describe a likelihood. Unlike the word 'most' which means 'more than half' or a 'majority,' there is no value attached to 'probably.'
Additionally, the two words mean different things. Probable (or probably) is used to describe the likelihood of something occurring whereas most is used to describe the quantity of something in amount or degree.
This passage states that most of the proposals endorsed by the Citizens League have been passed by city council. As the majority of these proposals have been passed, future proposals endorsed by the Citizens League will probably (or likely) be passed as well.
The passage is using the fact that the majority of these proposals have been passed to conclude that future proposals will also likely be passed.
Hope this has been helpful! Please let us know if you have any further questions.