I do think a lone wolf scenario is correct. I have continued to be surprised throughout the course of my life at how strong men are compared to me. Research has shown that basketball players are the second-fittest athletes, after dancers. I think Rudy was more than strong enough to handle Meredith, who was very slight.
There was a case in Seattle a couple years ago in which one man with a knife attacked and raped a pair of women for over an hour, using the knife to keep them under his control. One of the women died after breaking a window and escaping into the street. As she was dying, she said to one of her neighbors, "He told us he wouldn't hurt us, but he did. He lied."
And this is the most important point of all. People who are being attacked by someone armed with a knife are very often far more submissive and compliant that one might expect. This is due to two major factors: firstly, sheer fear often induces a mental and physical "paralysis" in the victim(s); and secondly, the victim(s) is/are usually offered a choice by the perpetrator, along the lines of: "Do what I say and you won't get hurt - but if you struggle or resist, I'll kill you". If something like this is said, then unless the victim(s) is/are very confident that they will be able to overpower the attacker before being injured themselves, the logical option is to believe that compliance is the least-bad option.
This is, for example, what happens in the vast majority of armed robberies on banks. We're all familiar with the cliched announcement by the lead robber as the armed gang storm into the bank: "Everybody on the floor. Stay where you are and don't move, and nobody will get hurt." And the same message is used to the bank workers themselves: "Put the money in the bag / open the safe etc - if you do what I say you won't get hurt". And in most armed robberies on banks, the customers and staff comply, and nobody DOES get hurt. The threat of using a weapon (usually a gun/shotgun in this case) is sufficient to obtain full compliance.
The same thing happens in many violent rapes, and also in robberies against individuals: the assailant uses the threat of extreme violence as a weapon in itself. And the majority of people will comply in such a situation - since however bad it is to be raped (or, to a lesser extent, robbed of all your valuables), it's vastly preferable to being killed.
However, the knowledge of this psychology can be to the benefit of sadistic sexually-motivated killers. The BTK killer in Kansas used exactly this psychology to enable him to control and tie up entire families, before torturing and killing them at his leisure (often killing the first victims in front of the other restrained members of the family). And a rapist with high levels of adrenaline coursing through his veins can react to a brief struggle from his victim by exercising the ultimate form of control over her.
So, with all this in mind, I think it's entirely feasible that a single attacker - Guede - had a brief initial struggle with Meredith, after which he got the knife to her throat and told her to comply otherwise he would cut her throat. And if you're going to restrain and control someone with a knife to their throat, the best (perhaps the ONLY) way to do this is from behind, since you then trap the victim's neck between the knife blade and your shoulder/chest.
So my belief is that Guede had Meredith in this position on the floor, and she was relatively compliant. This would have given Guede the opportunity to retain the knife at her throat in his right hand, then use his left hand to start removing the clothing from her lower body. I think it's then possible that Meredith realised what was about to happen, and either screamed and/or moved against the knife - and one or both of these things caused Guede to stab her in the neck. There's no need whatsoever (in my mind) for there to have been multiple attackers to have been involved - a single fit man with a sharp knife and a threatening demeanour can be more than sufficient to restrain and pacify a single victim (and often, as in your example and that of BTK, more than one victim).