1. If this is a prestigious international law firm, you have approximately a 0.001% chance of staying there 7-8 years. What level would you come in at? Unless they are offering you an Of Counsel or Staff Attorney (i.e., non-partnership-track) position, you should expect major "hard looks" as you approach partnership (ours are around the 5th-year associate level and then 7th year; we have a 7-yr partnership track, so may be different at a big firm with a longer track). So unless you integrate quickly and become irreplaceable, I would make the decision assuming you'll looking for work again by the time you hit 7th year associate.
2. Labor and employment is not a profit center for these firms, it is a client service. This means there are even fewer partnership slots (I also work in a specialty area, and people in my area at larger firms are frequently put on hold for partnership for several years, because all the "client service" lawyers are competing against each other for the one slot that opens up every year or two).
3. 1,900 billable: first, I assume that means 2,000 billables for those who are "really" on partnership track (so, again, if you want to survive the 5th and 7th year reviews). Second, I would assume at least 20% for nonbillable, if not more. Third, as a lateral, your first job is to make yourself indispensable to your new bosses, so I'd assume even more for the first year or two. Finally, the other thing many people forget is that the work is never evenly-spread over the course of the year, especially in these specialty areas -- you should assume that some weeks will be 60-hr billable weeks (+ any required nonbillable), and others will be 20 hrs and you're scrambling to find stuff.
4. Commute: a 1-hr commute home feels much, much worse when it's your 81st work-related hour of the week than when it's your 45th or 50th. To me, the job hours or the commute would be doable, but I have a hard time envisioning both together.
5. What does your fiancee think? What are her hours? Is she on board with doing more of the cooking, cleaning, homecare, etc., on top of her regular job? Will she resent that you're not around/not pulling your weight? If she isn't willing to up her own share of the chores, you're likely going to end up outsourcing more, eating more takeout, etc., because if you're getting home at 9PM, you're not going to want to sit down and cook. Be realistic about the additional costs (financial and personal) associated with the job. Fewer years to FIRE isn't a win if it costs you the person you want to spend those FIRE years with.
6. Do you guys want kids during the time you're at this job? If so, is your fiancee willing to shoulder that load as well? Discuss.
7. If you are looking at this as a 3-4 year cash infusion and do not plan to try to stay on past, say, 7th year associate level, that is probably a more realistic/feasible option (many things are tolerable for short periods). But in this case: what is your post-big-firm employment plan? Is there a realistic option for you to find another job in your current desired town after you spend 3-4 years building relationships and connections in another one? Seems like local employment options are limited, given your current $48K salary. If your top priority is staying in that town, I'd make sure there was actually a bush around before leaving your bird in hand.
Personally, there is no way on God's green earth that I would accept that kind of offer with that commute, unless I truly had no other option to pay my bills (and even then the first thing I'd do is look to move). The job may accelerate your time to FIRE, but it will also accelerate your desire to FIRE by more than 2x.
Then again, YMMV -- I used to do the Big Firm thing, followed by the smaller-firm-but-train/subway-commute-into-DC thing (leave at 6-7 AM, return home at 7-8:30 PM). Neither was livable or worth the money for me. So I chose a smaller, more specialized firm with a better work-life balance in a mid-market, mid-cost city. You are younger and hungrier than I am now, so you may decide differently -- just make that decision based on all of the facts, not just the big fat salary number. It's really, really hard to envision what 13 hrs/day of work/commute feels like until you're in the middle of it.