In S01E11, Francis kills Peter which was then later assumed to be a suicide. How come Francis got away with this? Why was there only a handful of people asking questions? Why did no one really question the fact that Peter has been found in the passenger seat? This all just doesn't add up. Francis wasn't even a big deal back then. He was neither protected nor powerful. And he was very close to Peter - yet he hasn't been questioned about the death.
Because there is no reason to suspect murder. Yes, Russo had some more contact with Francis Underwood 'recently', but in the political circuit it's not abnormal for politicians to extend their network. It's certainly not abnormal for more 'senior' politicians to guide/foster more 'junior' politicians, so nothing suspicious there. Underwood's low or high position has nothing to do with that, though I wouldn't call the leading majority whip a low position (but I'm not an American so might be mistaken there).
Aside from that a major scandal about alcoholism just surrounded Russo, after which he has continued drinking and shown to be erratic. Even more, it was not new but a relapse. There is enough to suggest he was in a very bad situation, which is sadly not uncommon for people to commit suicide.
Your only point might be the passenger seat, but even that is not that weird. In essence there are two steps to this: starting the car and closing the garage door. It's not far-fetched to assume he came in driving, never stopped the car, got out to close the garage door, and got in on the passenger seat again. Yes, it may be a bit weird, but no, it's not that huge a deal to jump to murder given all the above, certainly given that he was intoxicated (a simple test can confirm that).
Yes, this seems like a 'perfect' murder, does that make it realistic? No, because in real life finding a perfect target like Russo that is gullible, has few close relatives, has major personal issues mas still has extreme professional success, while reacting extremely desperate to an 'assault'/scandal is extremely unlikely.