1. If by trade deals you mean "Trade Agents", then yes, you only need one per station. They'll keep your trading computer up to date with station offers. What you can do multiples of is get various discounts. Some are unlocked via chat mini-games, others via hacking a trade terminal and station scanning, which is...
2. ...what you do when you get close to those "i" icons, and trading port icons (that have + or - on them). Sometimes those unlock special offers (cued by a sound and a short message on your radar monitor). Those offers stack up with discounts you get from NPCs. Once I've "discounted" myself so much that a station started paying me for buying their product

(I don't know if this has been fixed since).
You can also get missions to scan a particular station to XX %. The percentage goes up with each "i" scanned. Scanning is somewhat easier if you put your scanner into "Station Scan" mode, which would highlight already scanned modules both in the cockpit and on the radar.
One other benefit of scanning is getting production info into your encyclopedia. But for that not only do you have to scan a production module, you should also open it's information and actually look at the production cycle.
Some stations can only be scanned to 100% using a (illegal) drone.
3. You can't trade commodities with the Skunk, no. What you can trade is "inventory" items that you pick up or craft. You can buy and sell these via remote trading so as to not dock to stations every time.
4. Scanner does not affect station scanning range, that can only be increased via mods.