The interaction between timeshare holders should be detailed in the individual contracts. Refer to the contract and consult with the management or HOA.
Yes. You do not trade directly, rather you use an organization to facilitate the trade. Your property is given a rank and you can trade for equally ranked property
Timeshare owners can share with anyone they want, as long as they follow the rules of the timeshare. They might need more timeshares for a bridal party, etc.