question number one is what the existing Private Domains are used for. See below.
If you have a "sendout" Private Domain, the Delivery profile - not the sender profile - manages which IP you warm up. So you are free to combine domain and IP, although keeping the setup stable during the warm up is recommended.
The following is largely sourced directly from Salesforce documents (which explains the "we" in the text) and should help answer your questions.
The relevant part is the second half (I included a divider to mark the spot), but I included the entire text so nothing is taken out of context:
What is the difference between a Private Domain that is included with the Sender Authentication Package (SAP), versus a Private Domain that is purchased separately?
A Sender Authentication Package (SAP) is a "branding" tool. A Sender Authentication Private Domain includes link-wrapping (i.e. links, images, and anything else that would normally reflect Marketing Cloud will reflect the chosen Sender Authentication domain instead when you hover). The domain is also authenticated with SPF/Sender ID and DKIM/Domain Keys.
SAP is a bundle of products (Private Domain, Dedicated IP, RMM and URL wrapping)
URL wrapping (commonly referred to as “account branding” is only available through a full SAP configuration)
There is a limit of 1 SAP per MID/account which means an account can only be “branded” with 1 domain (if a client needs to have unique branding for multiple lines of business then they’ll need multiple MIDs with an SAP per MID)
The SAP SKU (==product) cannot be broken into parts (i.e. we’re unable to simply pull the Private Domain out of a purchased SAP SKU)
A private domain is used as an authentication tool. A Private Domain that is purchased separately for sending is an authenticated domain for use in the FROM address only.
It is set up either via a User or Sender Profile, and this type of Private Domain DOES NOT include link or image-wrapping. It has an MX record, SPF/Sender ID and DKIM/Domain Keys.
While a single account MID can support only 1 SAP for branding purposes, there can be multiple domains in an account.
Private Domain is a single domain configuration (as opposed to a bundle of products). A single SKU (purchased product) can be used for 1 of 3 options (1 Private Domain - 1 option):
Private Domain for sending – authenticated for sending mail (we’ll sign the domain with SPF and DKIM for use in a From address) (unlimited number of Private Domains for sending per MID – one Private Domain SKU for each domain)
Private Domain for Landing Pages – the domain chosen can be used to uniquely brand landing page URLs in a client account (limit 1 landing page domain per MID)
Private Domain for Cloud Pages – the domain chosen can be used to uniquely brand Cloud Page URLs in a client account (unlimited number of Private Domains for Cloud Pages per MID – one Private Domain SKU for each domain)
The DNS records needed for each of these options varies which means a single Private Domain SKU cannot be used for both Landing Pages and Cloud Pages. (e.g. if your Landing Pages domain is pages.domain.com, the Cloud Pages domain cannot be pages.domain.com - they must be different)
hope this helps!