Welcome to Hell Unleashed!
I haven't personally built a bladesin but I can provide a bit of information that should help you out. In no particular order -
Dragon Tail will not help you deal with physical immunes because it converts physical damage to fire damage. In other words, a physical immune monster would receive 0 physical damage, and Dragon Tail would then multiply that by X%; math shows us that 4000% of 0 is still zero, so you would do no fire damage.
Because of that (and for the nature of the character) it may be worth pursuing Venom, but there is a disagreement among the HU community as to Venom's worthiness as a source of damage.
Warrior is probably a better bet to max because of its skill levels, equipment possibilities, and life/resists.
For survivability's sake, you'll probably want to max Fade (especially in HC).
As for a merc, I'm not sure what to suggest - you'll already be doing physical damage, so an A1 merc might not be the best choice. On the other hand, having a tank might not hurt, a lá Act5 wolf or Act2 prayer.
I think there are a few other bladesin posts floating around, so while you've said you're read a few posts here and there, it may be worth searching for those as people with more experience than I probably added their $.02