1) The amount on line 51 of the IRA Information Worksheet comes from user entries of ordinary Roth IRA contributions and from basis rolled over to a Roth IRA from a designated Roth account in an employer plan (a 401(k), 403(b), governmental 457(b) or the federal TSP) via Forms 1099-R entered by the user.
1a) No, entries of traditional IRA basis have nothing to do with any amount on line 51.
2, 3) As indicated by NCperson above, you no longer need to know the basis in Roth IRA contributions and conversions to determine taxable amounts of distributions from your Roth IRAs. These distributions are now tax-free, qualified distributions which are not reportable on Form 8606.
