@BilboRoth: Happy to help. I figured a lot of this out using the forums, so am just returning the favor. On your questions:
(1) I'm not an expert on form 6251, and go strictly by what TT tells me in their on-demand guidance. In forms mode, if you right-click (windows -- use whatever the Mac equivalent is) on any field there's an option for "Data Source". Clicking that will give you info on where TT thinks the data for that field should come from. Looking at line 18, it pulls from the 6251 Wksh, line 5. And if you look at the values that feed into line 5 you'll see that its the sum of line 1,2, and 4. Lines 1 and 2 are just for non-passive values. And line 4 is for "an activity for which the partnership interest basis limits apply, for an activity in which the taxpayer is not at risk, or for a tax shelter farm." (this comes from right-click and choosing "About AMT Worksheet..."). So for my MLPs, TT seems to be handling it correctly by routing the info through a different part of the Worksheet in the 'Passive' section. But if you feel its not right for you, it looks like values could be entered onto line 4 to flow through to line 18 (though you'd have to back that amount out of the next 'Passive' section on line 3).
(2) In Form mode, TT offers a "Blank Form". I use that to type in the IRS 751 info (and also the IRC 754 info when its required). Unfortunately, that form isn't submitted when you efile. The only advantage to using it is that it keeps all relevant return info in TT and part of the same file. There is no way to efile and also include the 751 info.
(3) 4797 is just a summary form, pulling from various other areas of TT (you can see this with the same "Data Source" trick). If you have the information needed to populate line 10 (I've never seen a MLP K-1 that provided it), and feel you need to, you'd have to remove the info from your K-1 and find the appropriate worksheet to enter it onto. I've never done this. Note that the assets that produce the Ordinary Gain reported by the MLP have nothing to do with the dates you purchased or sold the MLP. The MLP computes it (using every asset they own, its fair market value on the date you sold, any depreciation they've taken, etc) and provides a summary number to you.
**Say "Thanks" by clicking the thumb icon in a post
**Note also, I'm not a Tax Preparer/CPA. Just a volunteer, seasoned, TurboTax user.
Use any advice accordingly!