How to deal with an offlane Darkseer early game?

Further note that I'm not looking for advice on counterpicking (I realize a support with two reliable stuns would help a lot here...): I want to know what to do when I end up in this situation.

Uhh... that kind of depends on the heroes you end up with. I can't predict the future so I won't know your full array of options. Ranged support and melee carry covers a very wide variety of heroes, the main decider being "does one of you have a reliable stun or silence?"

However, if you can't farm consistently in your lane as a melee carry, your objective is pretty much always the same: build enough items to farm jungle only, and then do that instead. In PA's case, probably just rush Morbid Mask and get a Stout Shield if you don't already have it. You can lifesteal off of jungle camps, which do less damage than the ion shell, then return to lane to farm a couple of waves when your life is full, then return to jungle when your health gets lower again, etc. Your Lina should be able to hold the lane by herself fine against a solo Dark Seer. This is good for Lina, because if she can use the gold she gains to buy a Eul's Scepter, then you can get kills on Dark Seer when he tries to push the lane again. He can still put ion shells on incoming creep, but you will be able to zone him out of XP range.

So, important general things:

• Get reliable disable via an item choice if you don't have it already
• Farm woods if you don't have anything that allows you to simply sustain the ion shell damage

Another thing you can do is have Lina fight elsewhere on the map, always when you have greater numbers in the fight. This will prompt their team to either lose the fight, or group up as 5, and if Dark Seer leaves his lane then PA can free farm for a few waves. This sounds like it requires coordination, but if you buy a smoke and ping it enough times in woods near their lane people will eventually get the idea.

Well, it's surprise to me, but noone reminded that you can go on Iron Talon if you see Dark Seer in pick - it's good item on farming jungle, but it's good to kill ion shell creeps effectively. I agree with others about regeneration problem - so here the dilemma how sure you are about your health. Currently, it's ok to buy Vanguard on many carrys and do not force Battle Fury (example: Antimage). So you might start with items like Tangos, Healing Salve (to regenerate after first two packs), Ring of Protection and Recipe for Talon. Notice: you don't need Talon on first level - creep wave will be with two Ion Shells, so it doesn't do the thing. Than you can buy Stout Shield and carry another Salve to yourself. In this build up there is only one problem - it's last hitting first two-three waves. You need to practice last hitting in lobby agains Dark Seer, and you will see that's after 4 lvl you are ok with it.