According to Microsoft's compliance website, Microsoft products are 21 CFR §11 compliant so if you are in the US, this is important. You can supposedly request their compliance reports through your Microsoft representative. It looks like Microsoft Approvals runs through Microsoft Teams. I've never used that functionality before so I cannot comment on any specifics. I would recommend following your internal software qualification procedures to assess the risk of the product and determine what validations may be required.
ISO 13485:2016 doesn't state anything specifically related to electronic records or signatures so you'll just need to complete software validation. Also, consider how visible software version changes are. You don't have control over the Microsoft product so make sure you won't encounter a surprise software version change that requires re-validation or an assessment of why re-validation isn't necessary. I'm not an IT professional so I don't know if there is any "behind the scenes" space that this is more visible or not.
As for the CAPA question, it does make sense to have some additional approvals at strategic stages of your CAPA. Otherwise, one person may complete the entire CAPA, including implementation of actions, and overlooked something, potentially that one of the corrective actions had an adverse impact on the product or QMS. This must be verified before actions are implemented so it definitely makes sense to have at least someone else review at this point. The other stages don't have the same level of risk, but it never hurts to have at least one other person review to ensure you correctly scoped the problem, your root cause analysis was thorough and complete, your corrective/preventive actions make sense (and do not have an adverse impact on the product or QMS!) and that the verification tasks were appropriately defined. These gate approvals don't need to involve everyone who signs for CAPA closure, but ideally should be someone with familiarity of the process or perhaps the owner of the process/product impacted (if this isn't the CAPA owner). A second set of eyes on things never hurts!