Plans are not monthly subscriptions.
You buy the amount of hours in the package and can use them whenever you like.
The time only decreases when your plan is actually running/boosting hours.
Adding more games does not decrease the time faster.

Yes, You can hour boost multiple steam profiles at once.
However, if you want to boost multiple profiles at once, you'd need a plan for each.

Once your Free plan has run out of time completely, You can click the "Renew Time" button on the right hand side and you will get 100 new hours refilled instantly.

Yes, You can transfer the time from any paid plan to any other,
You can do this over on: User > Settings > Advanced Controls > Manage

Display a custom "In-Game" status on your steam profile and in the friends tab,
You can also use emojis, get creative.

Reply to any incoming messages with a custom automated message for when your plan is running.

Set your custom appearance and status on steam, such as Offline, Away, VR, Big Picture, Etc..

Hides the 3 most recently played games on your steam profile while keeping the last 2 weeks playtime visible.

When you start a game your self on steam, your plan will pause, using this feature it will auto resume for you.

Automatically accept any incoming friend requests when your plan is running.

Automatically stop your plan after the set amount of hours, useful if you want to gain a specific amount of time.

Farms all remaining card drops for your profile.