@Bossy Stick to player 1, in general it is only worth increasing it for xp.
I fully agree to that. Random bosses and those like Pindle won't drop more on higher /p settings.
Insight won't really help your orb side, only the fire side. It might break a few immunities, but hardly necessary for a dual-tree sorc.
You certainly mean Infinity, right? I also suggest not to use Infinity.
Insight is the better choice because on /p1 you are typically using fireball instead of meteor and fireball depletes the mana bulb very quickly. You probably don't want to waste time with looking for mana potions all the time. That's pretty annoying and it costs time as well.
If you need better items (not runes) you should run meph (reliably up to TC75) or pindle (up to TC87). See
https://diablo2.diablowiki.net/TC_1.10
When looking at his gear, he already has what's best provided by Meph. I think he needs to run the Ancient Tunnels in addition to Pindleskin.
You didn't list your current armor, but a smoke runeword (nef+lum) gives 50 all resists.
Before having Tal's armor, I would rather wear Skullders, for its massive MF bonus. If that's not available as well, then it's Wealth or at least a 4 ptopaz armor.
You can also find resists in the glove slot, boot slot, and ring slots from rares.
I agree, except for gloves. I would wear Magefists here.
Also resist charms. For your merc a treachery runeword (shael+thul+lem) will boost his damage, but unfortunately the fade proc won't be reliable for short runs such as pindle or meph.
When doing the Tunnels before Pindleskin, fade has usually been activated.
BTW, Mephisto is best visited without a merc becausde he would spoil the moat trick (luring Meph to the moat, teleporting to the other side and bombard him with meteors from far away) and also stir up monsters which could be ignored otherwise. After that, you can also go to Andariel quickly by entering the portal to act 4 where it's a short way to the waypoint. A meteorb can simply kill her in a direct confrontation, with a couple of fireballs. At /p3, Meph and Andy drop 6 items almost all the time.
Rhyme Shield
Tal Ammy/Belt
Waterwalks
Chancies
Couple crappy rings
I suggest to wear a Spirit shield together with the Occy, not Rhyme. Apart from +2 to skills, Spirit adds resists, vitality and (very important) lots of FHR which is very important when teleporting around. Put that Rhyme into the weapon switch, together with Ali Baba's. That's what you could use in the Tunnels and versus Pindle BTW.