RE: Sockets -
Chest armors and 2H weapons are the only slots capable of being 6-socketed. Helm/Gloves/Boots can have a maximum of 4 sockets, 1H weapons and shields have a maximum of 3 sockets. The number of sockets that can be ROLLED (i.e. drop sockets or rolled with a Jewelers Orb) is based on the ITEM LEVEL of the item (hold alt over an item to see this). ANY 2H/Chest of item level 50 or higher can roll 6 sockets. Anything 35+ can roll 5 sockets. I think the requirement for 4 sockets is like 20. An item can always have any or all of it's sockets linked (so if it can have 6 sockets, it can 6-link).
HOWEVER, even if your item doesn't meet the item level 50 requirement, the master "Vorici" provides an Artificer's Table for your hideout, which can be used to FORCE 6 sockets/6links. This is NOT cheap. Usually (but not always, RNG is RNG) it is cheaper to "manually roll" your sockets with jewelers/fusings than pay the Vorici rates. Personally I hate gambling though...
So yes, your Infernal Mantle can be 6-linked, if you're willing to spend enough to do so.
RE: Divination Cards -
If you aren't farming for anything in particular, it's highly unlikely you will be *able* to "cash in" any cards except maybe Carrion Crow or The Scholar. When you get to mapping, The Gambler shows up moderately often. As I stated before, Carrion Crow is GODAWFUL, it rolls a random base item and a random life roll, resulting in a completely useless item almost every time. The Scholar is pretty worthless unless you're playing a magic find character - I dunno about you but I always wind up having hundreds more wisdom scrolls than I actually need.
RE: Farming for loot before Cruel -
DON'T. You're wasting your time. Act 4 Normal is MUCH harder than Act 1 Cruel. By the time you'll actually be challenged in Cruel, you'll be in 50+ zones where your farming can result in much more valuable drops, such as 6-socket items which can be vendored for 7 Jewelers' orbs.
RE: Flame Totem Links -
Unless you have really high crit chance (hint: you don't right now, you aren't high enough level) drop the crit damage gem for something better. Added Chaos would fit in nicely without needing to change socket colors, Fire Penetration would be even better but would require a red socket instead of the blue you have currently.