It sounds like someone else claimed your kids last year and you've already been in touch with the IRS about it, with that investigation still ongoing. (If I read that wrong and you have not been, you can still amend that return to claim them and get a refund (that's in the IRS's own words here) after the IRS sorts out who's actually entitled to claim them.) The second (and subsequent) e-filed return with the same dependent Social Security number on it will always be rejected, resulting in the need to mail the return in, and the process (discussed in this TurboTax article) with which you may already be somewhat familiar.
The youngsters turning 18 doesn't change anything, if you're otherwise still entitled to claim them (here's the IRS rules for Qualifying Child and Qualifying Relative). Note that when they turn 19, they can no longer be a "Qualifying Child" unless they are continuing with post-secondary education.
Regarding the IP PIN, you must generate a new one every year (as discussed here). You can call 1-800-908-4490 for specialized assistance if you're having trouble with that. Good luck with this, @partridge9025!
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