Do multiple sources of counting as one size larger for carrying capacity stack?

I am currently mocking up a Goliath Barbarian, and was wondering if there is a limit to the amount of sources of “You count as if you were one size larger for the purpose of determining your carrying capacity” that stack.

From prior browsing, I’ve found that Goliath’s Powerful Build & Totem Barbarian’s Bear Aspect feature stack in that regard, but could you stack, for example:

  • Powerful Build (innate Goliath feature)
  • 6th level Totem Barbarian Bear Aspect
  • Brawny feat

Essentially, could you double your carrying capacity and lift/pull capacity thrice?

Can a Light Domain Cleric cast Burning Hands while carrying a shield?

I have a Light Domain Cleric who gets, as part of the Light Domain, the Burning Hands spell. Can I cast this spell while carrying a shield?

I subscribe to the idea that one physically must touch one’s thumbs together and spread one’s fingers per the spell as the somatic component of the spell. Is it reasonable to think that a Cleric with a shield strapped to their arm is able to make such a gesture and not burn their own shield?

Does a sprite familiar’s equipment count against its carrying capacity?

Under Lifting and Carrying (PHB 176) it says:

Your carrying capacity is your Strength score multiplied by 15. This is the weight (in pounds) that you can carry… You can push, drag, or lift a weight in pounds up to twice your carrying capacity (or 30 times your Strength score).

and also specifies that a tiny creature can carry half as much

A sprite familiar summoned through a warlock’s Pact of Chain feature has a strength score of 3 and so has a carrying capacity of 22.5 pounds and a push, drag, lift limit of 45 pounds. The sprite stat block also specifies, however, that they wear leather armor and carry a longsword and shortbow.

Supposedly these weigh 10 pounds, 3 pounds and 2 pounds respectively but considering they are smaller than those worn by a medium humanoid it is unlikely that they weigh as much.

Is there any official guidance as to how much the sprite familiar’s equipment counts against its carrying capacity?

Is there any way to gain the endless special quality without carrying around a small necromantic magic item?

Dragon Magazine #354 has a fairly well-known special quality called endless, which prevents aging and all its normal effects, but is unfortunately not actually granted by its associated feat, Wedded to History. Within the pages of this magazine, the only way to gain the quality (DM fiat aside) is to have someone cast kissed by the ages on you, and then to forevermore give up a magical item body slot and risk taking a penalty if you ever lose the item–which also radiates enough necromancy to make many NPCs very uneasy.

But what about methods outside the pages of the magazine? Is there any sort of feat, feature, or other special means by which someone can gain this extraordinary special quality, without needing to go around holding a pseudo-phylactery?

Carrying Capacity and Spells

So, as I go into detail on the spells I noticed something that I hadn’t noticed before. For example, with Fly, it states:

The subject of a fly spell can charge but not run, and it cannot carry aloft more weight than its maximum load, plus any armor it wears.

I had always assumed that armor weight counted toward the carrying capacity and so it often pushed characters into carrying a medium load on its own. However, this implies it might not. In the starting of the Adventuring chapter, however, it explicitly states:

If you want to determine whether your character’s gear is heavy enough to slow him or her down more than the armor already does, total the weight of all the character’s items, including armor, weapons, and gear.

Are one or other of these a misprint or am I missing something in understanding why it would say the gear plus the weight of armor? Can encumbered characters become unencumbered by a flight spell, if subtracting the weight of their armor makes the difference, for example?