1. How do I get Moderators to be able to view (and possibly) edit a users subscriptions from the frontend?

You can go through the process of starting a subscription using the Subscriptions tab while viewing a user other than yourself, but you won't be able to just give them the subscription for free without a coupon, using maybe a dummy payment gateway, or using offline gateway then marking the payment as paid from invoices page. Typically you should just be doing this from backend.

The tab seems to only show for the user (view and edit) and allows the user to renew/unsubscribe etc. But I can't get it to show for moderators on the front end.

Your moderator usergroup likely doesn't have permissions to do so then. You can adjust CBSubs permissions in Settings then click the Permissions button on the top right. They'll need "View User Subscriptions" and "Manage User Subscriptions" permissions.

2. Is there a way for moderators to edit subscriptions for users (frontend or backend)? Specifically adjust the expiry date to extend a subscritption if necessary?

This can only be done from backend within CBSubs > Subscriptions by clicking the # under the ID column. You however should not be doing this unless absolutely necessary. Subscriptions should be extended via renewals allowing CBSubs to handles any and all necessary checks for duration, status, etc..