first of all, relax! The UK visa process isn't usually horrendously painful and long for Americans.
If she's coming back to the UK on a Tier 1 visa, then once you decide to get married she'll need to get a Certificate of Approval in order to marry you in the UK (takes about 4 weeks to get.) Then you give notice and get married as normal, and then she can apply to remain as a spouse if she wishes, using form FLR(M).
Alternatively, once you've been living together for two years and have the documents to prove it, she can apply for an unmarried partner visa even before you marry.