Definitely the easiest is if you can personally pay for the room and then your employer can reimburse you for the charges. And if they aren't forcing you to use a specific credit card, you can even use the Premier Marriott Visa and get 50% more points on top of the base 10 points per dollar. Since you are already a Marriott Silver member, you already know to call and make sure your MR number is a part of the reservation.

Enjoy your stay, and if you end up staying upwards of that 90 days, you should easily have your platinum status when you finish. Also note that basically, RIs don't give much extra in terms of benefits for Gold or Platinums, so even if you don't receive your status upgrade until the end of your stay it really won't affect you too much. (Though most of the times I have had long term stays, I have been asked to "pay" the bill every month or so. So you may still actually see points and your elite status increase before you check out at the end of your stay.)