In the Tiberian tradition, a shva is pronounced as a short patah. In לַקרן, since the ק is gemminated, the patah is shortened anyway. Therefore the only difference would, in fact, be the gemmination. See pp.315-316 https://books.openbookpublishers.com/10.11647/obp.0163.pdf
According to other vocalisation systems, it would be different, but since, according to the Zohar, the Tiberian system is Sinaitic, the instruction to pronounce the word with a patah instead of a shva is meaningless unless the intention is to gemminate the ק, but if that is the intention, they should say that. Probably the kabbalists are just confused.
To push back a bit; even if kabbalists agree that this was the Tiberian tradition of pronunciation, people nowadays make a distinction between patach and (not so much chataf patach but) sheva. Therefore, the distinction encodes meaning. The way Shinuyei Nuschaot seemed to put it, the kabbalists are makpid. But that does not mean that they think there is a kabbalistic imperative to pronounce the Mishnah in a particular way, or that this is necessarily the historic way it was pronounced. Rather, it is a way to express the meaning in a particular way. Language is used to express meaning, and for listeners to understand that meaning. So kabbalists could be makpid now, because they want to show how (especially in the more likely option [2]) the statement should be understood -- that there's a break at lakeren.
In the Tiberian tradition, a shva is pronounced as a short patah. In לַקרן, since the ק is gemminated, the patah is shortened anyway. Therefore the only difference would, in fact, be the gemmination. See pp.315-316 https://books.openbookpublishers.com/10.11647/obp.0163.pdf
According to other vocalisation systems, it would be different, but since, according to the Zohar, the Tiberian system is Sinaitic, the instruction to pronounce the word with a patah instead of a shva is meaningless unless the intention is to gemminate the ק, but if that is the intention, they should say that. Probably the kabbalists are just confused.
To push back a bit; even if kabbalists agree that this was the Tiberian tradition of pronunciation, people nowadays make a distinction between patach and (not so much chataf patach but) sheva. Therefore, the distinction encodes meaning. The way Shinuyei Nuschaot seemed to put it, the kabbalists are makpid. But that does not mean that they think there is a kabbalistic imperative to pronounce the Mishnah in a particular way, or that this is necessarily the historic way it was pronounced. Rather, it is a way to express the meaning in a particular way. Language is used to express meaning, and for listeners to understand that meaning. So kabbalists could be makpid now, because they want to show how (especially in the more likely option [2]) the statement should be understood -- that there's a break at lakeren.