Change Default Email Setting

I have 2 email accounts on TB. Well, 3 if you count the Plusnet one being set up for IMAP and POP3. This email has been my main one forever, but Plusnet have sold it off to an independent company, which I've opted out of as it will become a paid service after 2 yrs. Both IMAP & POP 3 are set as my default email & I want to change it to my Yahoo email that I only set up on POP3. My 1st Q. is: I can see the red delete button next to the default notice in Settings. Does this just stop the email being my default or will it delete the account entirely? I don't seem able to make the Yahoo account my default without undoing the default settings for Plusnet first (makes sense). But just want to be sure. Q.2: Since setting up Yahoo email on TB, I noticed that while all my Inbox messages remain when downloaded on my PC, the Sent messages are being deleted to leave only the most recent (currently) 11 messages, like it is if I log in online to Yahoo webmail. Does this mean I need to configure it to IMAP as well to make sure nothing is deleted from my PC? Eventually, I want to delete the Plusnet accounts entirely from TB. Although I am still receiving emails sent to this address, which is still quite useful, I cannot use it to send or reply to these emails. Also, since the sell off I've been receiving unprecedented amounts of spam/scam emails, especially to my iPhone (TB isn't set up on my phone).