I graduated last year. Things may have changed since I went through, so current students might want to comment on any changes, but I can at least tell you how things were for my class.
The Dean's letter deal is something like this: You're sent a link to an online form to fill out. The questions are things like which clubs you were in, what other extracurriculars you were involved in, volunteering hours, leadership positions, and some other things. I'm not sure where it goes from there, but that information is used to build your dean's letter. So you don't directly write it, but I assume it's built from what you enter and edited from there. I heard anecdotes from friends that at residency interviews that they were told how useless our dean's letters were, but unfortunately that's not unique to the LECOMs by any means. I can't remember anybody mentioning it when I was interviewing.
I assume the second question is about 3rd & 4th year rotations. Rotations at LECOM are pretty well-documented if you search through this thread and others. There are good and bad rotations, and much of the responsibility for your schedule is put on you. You're given a list and allowed to put in your preferences along with 10-14 other students who share your rotation schedule (i.e. IM this month, surgery that month). A schedule is sorted out from those preferences. From time to time, you may find out you're dropped from a rotation for a variety of reasons, and you have to find another. Usually, the office staff can tell you, "xyz location has a spot" for whatever rotation, at least for the core rotations, and they will put you down for it if you tell them to. The times this happened to me (twice at the start of 3rd year, once late in 4th year), I was able to sort it out within a few days. There were definitely horror stories of people finding out a day or two before their rotation that a location cancelled their rotation... and it sucks. But the people I know who had that happen still got through, maybe with a rearranged schedule. Year-long spots tended to be stable with less drama and fewer dropped rotations, at least from what I heard. Depended on the location, though.
My rotations were uneven, but in general I feel like I was able to put together rotations in 3rd and 4th year that worked for me and prepared me for what I wanted to do. I can also tell you that if you're trying to pick between DO schools, LECOM's rotation issues, sadly, aren't unusual. Talking to my fellow residents and to other med students on rotations from other schools (NYCOM, Touro, AZCOM, DMU, Western, RVU, and UNECOM), LECOM was average or slightly below average from what I remember.
Good luck to you.