Can you escape from Dimensional Shackles as easily as by polymorphing?

The intent of Dimensional Shackles (which block all kinds of teleportation or interplanar travel on their victim) is clearly to prevent spellcasters from escaping.

But what happens if a Dimensional-Shackled spellcaster casts polymorph on himself (and then drops his concentration)? Is he then free from the Shackles and thus able to teleport away? Or are the Shackles somehow linked to him and will still be applied to him, still preventing him from teleporting/planeshifting/etc. until he actually escapes by succeeding on the DC 30 Strength check?

Polymorphing into a creature that is larger than the room

If a character has the ability to transform into different creatures (using polymorph or wildshape as examples), what would happen if they shift into a creature larger than the space they are in?

Example: a chartacter is in an 8x8x8 room and turns into an elephant via any shape changing spell/ability.

Would the polymorph or wildshape still take effect?

Would the character take damage and if so how would that be determined?

Would any other creatures in that room take that same amount of damage?

Can I get a paladin’s steed by True Polymorphing into a monster that can cast Find Steed?

I am examining niche ways to cast Find Steed. I’m specifically interested in ones that don’t require me to take levels in Bard, Thief Rogue, Wizard, or Paladin.

I had the idea that I could True Polymorph into a creature that has the Use Magic Device ability, or access to the Paladin spell list, and then cast Find Steed off of a paladin scroll.

background reference: What creatures can cast spells from spell scrolls?

Question

Is it even viable in theory that a character without access to paladin spells uses True Polymorph to turn into a creature who can use paladin spells, casts Find Steed from a scroll and then retains the summoned steed afterwards?

An important follow-up question: what is an example of a published creature able to use paladin spell scrolls? I want to satisfy the case where the DM insists that you need to have a published creature of some sort in order to cast True Polymorph.

Can polymorphing monsters spam their ability?

Some creatures (such as the Couatl’s “change shape”) have an ability that let’s them polymorph. I am wondering if this ability could be used again whenever the form is dropped to give the creature a massive effective health pool.

For example, if the Couatl used their action to change shape and become a Veteran, then was hit for enough damage to kill the polymorphed form, could it then re-use the change shape ability to transform into a fully healed Veteran once more?