hey hon!!! it is way too early to tell if a sac is viable or not!! you are not even 5 weeks and the heart doesn't even start to beat before 5 weeks simply because no human fetus reaches a stage with having a beating heart before full five weeks (and honestly, a heartbeat can't often be found before 7-8 weeks).
i would definitely wait a bit with medications, especially as nothing alarming was found in your tubes and you had no severe cramps so far. also, i would DEFINITELY get another doctor's opinion before starting miscarriage-inducing medications. i think your doc wants you to start metotrexate drug that is used to arrest the embryonic growth in case of ectopic pregnanices.
since they haven't found a sac in your tubes and you are really really early on (around 16 dpo so barely two full weeks since conception!!!) i would DEFINITELY avoid any interventions for now and get a second opinion.
i get that your doc wants to be extra safe to you as misdiagnosing an ectopic and having your tube burst would be a big issue for her, also potentially legally, but also giving you miscarriage inducing drugs and telling you your baby won't develop anyhow to make you take them is a bit of a lie and a bit of pushing it. she CAN NOT possibly estimate a viability of a sac at two weeks post conception.
she may think: "OK, she has four kids already and got her tubes tied so if she loses this one even if it's healthy it ain't so bad and it's less risky for her".
she doesn't take into account how it may impact you emotionally.
and honestly, a doctor that can't find your right ovary at all and has found nothing in your tubes while she has found a developing sac in your uterus, yet pushing you to take drugs should be double-checked in terms of diagnosis.
i really pray that it is all good with you, and i don't know why but something tells me it is all fine with you and your little bean.
get yourself a second opinion and then take a decision. if there is no trace of ectopic in your tubes, i wouldn't take a medication to miscarry. it is really really hard to tell the viability of a pregnancy so so early on and no doctor should tell you you are to miscarry certainly that early on.