I am usually a lurker only but decided to chime in as I experienced this 2x & won 2x
I live in Muskoka as well 30 Miles from you
My 1st step would be to form an association of concerned citizens As a named group you can be put on the mailing list to receive all minutes of any Council meetings concerning this development {which immediately stops a lot of closed door nonsense}
My 2nd step would be to contact Mr Russ Black a OMB specialty lawyer in Bracebridge he is simply the best & will tell you what you may have to work with {He also has worked with Mary Moroz an excellent planner from Port Sydney} She helped develop Haliburton and possibly Muskoka's "Official Plan" as well These initial steps won't cost a lot if shared by a group.
Don't be fooled by developers "big pockets" as sometimes it doesn't come to this. Town councils often overlook their own bylaws & an independent planner or OBM lawyer will find you the keys to fight with & sometimes stop it prior to any OMB involvement
Firstly look very hard @ environmental impact. If there is any water running through the property {especially standing water} Filling ponds, standing water or rerouting water flow is a big no no. Every development must have MOE & MOT approval & if you get either on your side they have instant trump cards on any development even over town councils.
STUDIES, STUDIES, STUDIES. Make sure that your council demands as many studies as possible from, impact, feasibility, Environmental, drainage, traffic, & damage to surrounding people & waters {Everything that you can think of} as it is an easy way for the council to appease disgruntled voters but insure if you go to OBM they have covered their A-SSSS
Lastly is a VERY IMPORTANT POINT Muskoka has an official plan which was laid out @ great expense to specifically afford for future development. To change zoning is going against their own Official Plan which OMB frowns upon. If it's a good idea now then it should have been in the "Official Plan" Your council may now have a very difficult time explaining why they want to go against the Professional planners they hired to develop Muskoka's Official Plan
You may be surprised what a determined, vocal & organized group can accomplish. After all Walmart was stopped in Bracebridge & there are no deeper pockets. They simply took the path of least resistance & built in Huntsville
I registered after 2 years of lurking & activated my email . Feel free to contact me if you have any questions that I may help you with.
Good Luck!!