- they're (only) for the Guild league tables - being high in the tables (currently) offers no reward other than bragging rights though
Yup that's one of the Perks you can pick for your Hero. Right now, picking a stat gain Perk would be better.
Correct - you get a type bonus for having three or four of the same type. - and the above Perk lets the Hero count the 'Giant' type keyword towards it.
My understanding (I haven't played Arena in an age) is that the troops are not from your roster - they are fixed at level 10 irrespective of whether you own them or not.
Only your Hero appears with your own stats and progression.
If I have this wrong, others can correct me.
Also tagging @LegendMaker if he's still around, as a master player of Arena.
Up to you what order you do stuff - but yes, Traitstones probably aren't the highest priority. Right now you just need troops and to level a decent starting team.
Save the Traitstones until you know what troops you will be using for mid game (post level 80-100 ish)
Do quests and challenges for easy souls.
Join a guild, as many said above.
Try and get Valkyrie or another troop that make souls - and then try and get something with the Necromancy trait unlocked.
Save gems to buy Armour for the loot bonuses - don't spend on cards until you have good Armour.