As I speculated on the comments, the best approaches might just be to either:
Use the Test.isRunningTest method to check for the test context and thus bypass the .setClientCertificateName method in your code.
Use a try/catch scope to catch the exception thrown when the platform tries to assign the missing certificate to the request.
The first approach ...
The premise of your question is incorrect. The current release of Salesforce is Summer '19. The release that is currently approaching public preview is Winter '20, which will be released in early October.
The certification exams do not include questions about releases that are not yet public.
I think the second problem you listed is causing the first problem, and unfortunately it's a bit of a layered answer. I'm also assuming that the MetadataService layer you've generated is all correct.
The first part to solve is getting the ConnectedAppOauthConfig.isAdminApproved = true. Now I couldn't get this to set on creation of the Connected App. However,...
I was able to query the Certificates using the Tooling API instead:
HttpRequest request = new HttpRequest();
request.setEndpoint(URL.getOrgDomainUrl().toExternalForm() + '/services/data/v46.0/tooling/query?q=Select+Id,DeveloperName+FROM+' + type);
request.setHeader('Authorization', 'OAuth ' + UserInfo.getSessionId());
Taking the summary out of comments for everyone's benefit: the screens you reference are for implementation of a scenario where an external service/app is acting as a SAML identity provider, SF is a service provider.
SamlSsoConfig is the metadata object that corresponds to the Single Sign-On Settings screen, it captures a configuration of the (external) ...
I'm going to assume you're referring to some form of web service and not browser certificates.
Most stacks/browsers/operating systems already manage the CAs for you so it's not much maintenance. In cases where you don't have a system that keeps the CA list updated then yes, it'd be the same amount of maintenance to change a cert out.
Most systems handle ...