I'm playing on Legendary and I'm trying to boost my defenses as much as possible. I will be taking advantage of enchanting/alchemy/smithing. I'm a Breton so I intend to get as high as I can in magic resistance/absorbtion as well, without using any stones.

1. Can any armor potentially hit the armor cap if you're maxing it out? Anything from Hide/Iron and up? Or is it only the higher level armor?

2. Does the Lord Stone have any effect if you've hit the armor cap, or does it stack with your defensive bonuses?

3. If you've hit the armor cap, does Oakflesh and spells similar to it have any effect?

4. Are there any other ways of boosting your physical defenses that I haven't mentioned?

1. Only if you use the resto loop glitch, otherwise no light armor will ever reach it, but I think steel plate and up will reach it if you have EVERY heavy armor perk and have the best smithing gear. besides that, only Dragonplate and Daedric (and possibly ebony) will reach max reasonably without it.

for the others, no. The cap is the Hard cap no matter what. 80% reduction is the absolute best reduction you can get, which occurs at around 567 AR.

1) Honestly most armors don't. Even the Orcish set can't reach cap unless you have perks in heavy armor; most light armor fare worst. But since you're enchanting/alchemy/smithing anyway, you might as well do the restoration potion exploit to get cap so you can wear whatever you want for roleplaying reasons and don't have to worry about wasting precious perks. (One in Juggernaut or Agile Defender is enough; save the rest for functional Armor perks)

2) Lord Stone gives you magic resistance as well, but physical cap is cap and it won't go any higher. The magic resistance stack with your Breton racial bonus as well.

3) Of course. They give you jack and squat.

4) You can avoid getting hit altogether by investing in Illusion and turn Invisible to snipe/slit throats.

Thank you. I'll go with Heavy Armor then.

Is it possible to have a armor cap without a helmet? Doesn't matter what type of armor it is but do you need a helmet?
I like having no helmet for RP reasons

You can reach armor cap with other armors if you are taking advantage of alchemy, enchanting, and smithing. You don't even really need to use an exploit. Use alchemy and enchanting to fortify your smithing as much as possible. That combined with light armor perks can get you over the cap. I have a dark elf that is over the armor cap with no helmet using dragonscale(didn't use any alchemy of any kind either)

That character does have ancient knowledge, but due to lack of helmet is also missing out on the perks that require a full set of light armor

With fortify smithing gear and a fortify smithing potion (and the relevant perks of course) plenty of armor can reach the cap. I don't even have all the perks and my glass and orcish are over the cap (I don't mean to imply you need glass, I wouldn't be surprised if iron can hit the cap even without the restoration exploit, and I'm sure elven can - but be sure to have the best fortify smithing gear and the best fortify smithing potion - made with fortify alchemy gear).

No helmet should be doable with dragonscale, dragonbone, daedric, ebony... You're going to lose some of the "full set" perks and a hood or circlet will be necessary if you want full enchants.

Thanks. I'll probably do Dragonbone