What message do you get when you use that command?

If you're not sure how to open the command prompt so you can enter the command...
http://windows.microsoft.com/en-us/windows/command-prompt-faq
The second Question "How do I get to a command prompt?"

Check your badges page to make sure you have drops remaining for those games. The yellow boxes in this image show what you want to inspect.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=503188239

You can idle it as many times as you want, although idling when you do not have drops remaining will have no meaningful impact. For most non-F2P games you only get half the set for free. Free-to-Play games do not have a set number of cards you can receive through playing, instead you get 1 card for every 9$US you spend on it.

If you're not sure if you have drops remaining for a game, check your Badges page.
https://psteamcommunity.yuanyoumao.com/my/badges/

Unfortunately, if you want the cards you need the playtime.

You would need to idle a game for 2 hours before normal drop rates start. After 2 hours of playtime you can't refund a game. You will render the game non-refundable before you get your first card.

Double check your Badges page to see how many of your games have drops remaining. The yellow boxes in this image show where to check.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=503188239

Free-to-Play games do not give any cards just for adding them to your Library, you must spend 9$US (or equivalent in local currency). Non-F2P games give half the set (rounded up) through playtime. Duplicates will count against the allotment of free cards. Trading/Selling/Using those cards will not grant replacement cards.

No, you must own the game to qualify. Family Sharing is borrowing the game.

The 2 hour mark is because Steam decided some games/accounts are required to have 2 hours of playtime before the normal timer starts, it was implemented to prevent abuse of the refund system.

Idling multiple games during the normal drop timer is at best just as effective as idling sequentially but most reports indicate a reduction (if not complete prevention) of drops when running multiple games.

It's possible, but best case scenario it will cause cards to drop slower. Worst case, it will completely prevent drops.

Idle Master reads http://psteamcommunity.yuanyoumao.com/my/badges/ and automatically builds the list of games to idle based on what games have drops remaining. You do not need to do anything other than signing in for that URL to work and have games with drops remaining.