Hello yourfur7,
Welcome to Microsoft Q&A forum.
Before proceeding to your questions, may I know what happens when you tried to install the mods? As far as I know, Xbox app on PC allow mods for GTAV Offline mode, even from third parties. However, the apps cannot guarantee the safety of the mod's files, so proceed as your own risk.
For whether or not the progress will be kept, the Online version will, since they are linked to your Rockstar account. As long as you sign in to the correct Rockstar account, your progress online is saved. For Single Player mode, usually it is not since the storefronts is different, you can consider moving the save profiles from one storefront to another, however this is not guaranteed to work.
To be sure if you are eligible for a refund of this, you will need to check on the request form. However, you said that this come from Microsoft Rewards, these usually acquired by code redemption and for most cases, code redemption cannot be refunded.
Please let me know if you have any questions.