IIS URL Rewrite: Match subdomain and redirect to HTTPS

Since you haven't posted your existing rule (the one that results in redirect looping), I can't tell you why it doesn't work. I can tell you what should work though:

<rule name="Rewrite all but one subdomain" stopProcessing="true">
  <match url="domain.com" />
  <conditions logicalGrouping="matchAll">
    <add input="{HTTP_HOST}" negate="true" pattern="^x.domain\.com$" />
    <add input="{HTTP_HOST}" negate="true" pattern="^domain\.com$" />
  </conditions>
  <action type="Redirect" url="http://domain.com/a/" appendQueryString="false" />
</rule>

<rule name="Rewrite domain requests" stopProcessing="true">
  <match url="domain.com" />
  <conditions logicalGrouping="matchAll">
    <add input="{HTTP_HOST}" pattern="^domain\.com$" />
    <add input="{PATH_INFO}" pattern="^/a/$" negate="true" />
  </conditions>
  <action type="Redirect" url="http://domain.com/a/" appendQueryString="false" />
</rule>

Here we have to rules, one to rewrite all requests where the url matches domain.com except for host headers exactly matching x.domain.com and domain.com (to avoid looping).

The second rule matches requests for exactly domain.com and to any other place than /a/ and redirects them if necessary

My solution would be use conditions for that. By matching the conditions against the {QUERY_STRING} you can use back references to use them in the redirect URL.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <rewrite>
            <rules>
                <rule name="Redirect" stopProcessing="true">
                    <match url="(.*)" />
                    <conditions trackAllCaptures="true">
                        <add input="{QUERY_STRING}" pattern="&amp;?(beta=[^&amp;]+)&amp;?" />
                        <add input="{QUERY_STRING}" pattern="&amp;?(gamma=[^&amp;]+)&amp;?" />
                        <add input="{REQUEST_URI}" pattern="^/redirect" negate="true" />
                    </conditions>
                    <action type="Redirect" url="/redirect?{C:1}&amp;{C:2}" appendQueryString="false" redirectType="Found" />
                </rule>
            </rules>
        </rewrite>
    </system.webServer>
</configuration>

The only possible problem with this solution might be (depending on what you want) is that the redirect will only happen if both the beta and gamma query string variables are present in the query string. If they are not, the redirect will not happen.

The redirect rule matches against any URL ((.*)). If needed, you can change that. I've also added an extra condition to not have the rule match against the redirect URL itself which would otherwise cause the redirect URL to be redirected itself.