Not sure on the exact chance for Life Leech to proc, but it appears to proc around 40-50%. Yes, the melee attack needs to successfully land (so opponents with incredibly high Wrest/Parry pose a problem for Life Leech pets).
When the pet inflicts Poison via Infectious Strike, yes, Life Leech can randomly proc from that same attack.
Any ability that increases the pet's melee damage output, particularly Chivalry and AI, will increase the effectiveness of Life Leech, since Life Leech heals for 40% of the damage dealt.
Necromancy for pets is pretty lackluster. The damage spells are fairly weak, the debuffs are meh, they waste mana on Conduit which doesn't even work for them correctly (doesn't chain their single target spells like it's supposed to), and use of Curse Weapon at low health is unreliable. For Chiv+AI Ossein Ram, you want a high % rated 2 slot one. Even then, they can't get 120 Tactics and max out around 590 Health.
Dread Spiders are overall lower intensity than Ossein Rams, especially since a huge chunk of their total Intensity is used up by Magical abilities that become pointless upon switching to Chiv. 1,500 in Magery, 1,500 in Necromancy (Ossein Rams also have the 1,500 intensity in Necro), 100 in Poisoning. Their 100 Poison Resist also means they can't max out Physical, Fire, and Energy resists at 80.
120 Chiv+AI Ossein Rams can deliver some truly monstrous damage when under Berserk and Provo Buffs, which in turn allows them to self heal for a ton. Here's a screenshot of my 120 Chiv+AI Ossein Ram (Zeliek) delivering a 436 damage AI to a Balron while under Inspire/Invigorate and EoO. He wasn't even Berserk yet (wasn't below 50% Health). That's a 174 self heal if Life Leech procced.
