How to create a redirect in the .htaccess file, with 2 exceptions

Try the following, before the existing WordPress directives:

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule !^(abc|abc-def-g)$ / [R,L]

This states that for any URL that is not /abc or /abc-def-g and which does not map to a physical file or directory (ie. your static resources, images, CSS, JS, etc.) then redirect to the homepage.

The ! prefix on the RewriteRule pattern negates the regex.

This is a temporary (302) redirect.

You don’t need to repeat the RewriteEngine On directive.

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UPDATE: If your URLs end in a trailing slash then you should modify the RewriteRule pattern:

RewriteRule !^(abc|abc-def-g)/$ / [R,L]

Or, make the trailing slash optional:

RewriteRule !^(abc|abc-def-g)/?$ / [R,L]

Another way of writing the same thing is (as you mentioned in comments) use RewriteCond directives to check the REQUEST_URI server variable instead of the RewriteRule pattern. This might be easier to read, however, it is marginally less efficient since the RewriteRule pattern is always processed first. For example:

RewriteCond %{REQUEST_URI} !^/abc/?$
RewriteCond %{REQUEST_URI} !^/abc-def-g/?$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ / [R,L]

Which is the same as (using alternation):

RewriteCond %{REQUEST_URI} !^/(abc|abc-def-g)/?$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ / [R,L]