Similar to membership changes within a single Scrum Team, adding or removing additional Scrum Teams working on the same product will impact productivity in the short term. Adding additional teams often go through recognizable stages as they change from being a collection of strangers to a united group with common goals. Bruce Tuckman's Forming, Storming, Norming, and Performing model describes these stages.

Explanation
The Sprint Review is an event that inspects the outcome of the Sprint and determines future adaptations. The Product Owner has the authority to release the Increment to production at any time, but this does not mean that the Sprint has to be stopped or changed. The stakeholders can provide feedback and suggestions for the product, but they cannot interfere with the Sprint Goal or the Sprint Backlog within the Sprint. The Scrum Team can decide to:
Continue sprinting but shorten the Sprint length to allow for shorter feedback loops (A), which is a valid option if the Scrum Team agrees that this will improve their agility and value delivery. However, this should not be done frequently or arbitrarily, as it may disrupt the rhythm and focus of the team.
Continue sprinting and include the customer feedback in the Product Backlog, which is the most appropriate option as it respects the empirical nature of Scrum and allows the Product Owner to prioritize and order the Product Backlog based on new insights and stakeholder needs.
References: : [Scrum Guide], The Sprint Review : [Scrum Guide], The Product Owner : [Scrum Guide], The Sprint : [Scrum Guide], The Product Backlog

Using Scrum does not ensure that adding more resources to a project proportionally increases the value delivered. In fact, adding more resources may have the opposite effect, as it can introduce more complexity, communication overhead, coordination challenges, and integration issues. Scrum values quality over quantity, and focuses on delivering the most valuable and feasible outcomes in a given timebox . Scrum also encourages teams to be self-organizing and cross-functional, which means that they can work autonomously and collaboratively without relying on external resources or dependencies.

As a servant/leader, the Scrum Master facilitates conversations through open ended Question:s in order to help the team members make the best possible decisions according to what is known at the time. He/she does not approve or reject team decisions but ensures the team stays within the boundaries of the Scrum framework.

User acceptance testing is a process of verifying that the product meets the user's needs and expectations. It is an important feedback mechanism that helps the Scrum Team validate their assumptions, inspect the value and quality of the Increment, and adapt their plans accordingly.
Scrum requires that the Increment be transparent, meaning that it is observable and understandable by anyone with a stake in the outcome. Scrum also requires that the feedback loop be short, meaning that the Scrum Team can receive and respond to feedback quickly and frequently. Therefore, it is not a good idea to have user acceptance testing done every third Sprint, because:
The Increment will not be transparent (B), meaning that the Scrum Team and the stakeholders will not have a clear and shared understanding of what has been done and what remains to be done. This may lead to misalignment, confusion, or conflict over the product vision, scope, and quality. The feedback loop is too long (B), meaning that the Scrum Team will not be able to inspect and adapt based on the user's feedback in a timely manner. This may lead to wasted effort, missed opportunities, or reduced value delivery.