1) you'll need to send in the updated 8606 for 2024 to record the cost basis, in similar threads it's advised to send a 1040X with it, see
https://ttlc.intuit.com/community/retirement/discussion/amend-tax-return-for-form-8606/00/3301099
https://ttlc.intuit.com/community/after-you-file/discussion/amend-form-8606/01/3428188#M767296
see also end here for prior year basis updates (tho this refers to missed 8606 filing altogether and states "Don’t amend your 2024 return to record your basis")
https://ttlc.intuit.com/turbotax-support/en-us/help-article/retirement-benefits/enter-backdoor-roth-...
2) yes update the cost basis in the interview question to reflect the correct carryover from 2024, which will go onto Form 8606 Line 2.
https://www.irs.gov/pub/irs-pdf/i8606.pdf
@dmertz @VolvoGirl @Mike9241 @DoninGA @mesquitebean
