(a) The Gemara says a bigger Chidush- that the Pasuk of Lo Sa'amod teaches that one is required to spend money to save someone else's life and it is not sufficient merely to make a personal physical effort to save him. However MINCHAS CHINUCH 237:1, that you cited, writes that it is possible that once the Torah tells us that one must hire rescue workers, it follows that one also transgresses the positive commandment of ve'Hasheivosa Lo if one does not spend the money and not merely the negative Lo Sa'amod. The Gemara could have said this but preferred to stress the practical ramification that one must lay out the money (see note to Minchas Chinuch who cites CHIDUSHEI ha'RAN in our Sugya who also seems to write that in fact one transgresses both commandments).

(b) I would like to suggest an answer to the question of MAHARETZ CHAYES Bava Kama 85a, why RAMBAM does not mention the Mitzva of ve'Rapoh Yerapeh. My answer is based on TAZ YD 336:1 who explains why, even though Gemara Bava Kama 85a only learns from ve'Rapoh Yerapeh that the Torah gives permission to the doctor to heal (and that he is not considered as denying the Heavenly decree that the patient should become ill), nevertheless SHULCHAN ARUCH YD 336:1 extends this and writes that the doctor actually has a Mitzvah to heal. TAZ cites Berachos 60a that really people should not take resort to doctors and healing but instead should trust only on Hash-m and rely solely on praying for a recovery. However, since most people are not on such a high spiritual level, the Torah gives us permission to go to a doctor. Once the patient comes to the doctor, the latter indeed has a Mitzvah to heal him but Rambam did not mention this because this is not the primary element of the Torah's outlook on healing. (Rambam usually only includes obligations which are mentioned explicitly in the Gemara whilst Shulchan Aruch more frequently mentions practical Mitzvos which are not necessarily stated clearly in the Gemara.)

(c) According to the above, Ramban can also be understood. Ramban never suggested that if a patient comes to a doctor the latter should send him home and tell him to pray to Hash-m instead. Ramban's complaint is only about the patient who should not have come to the doctor in the first place. All 3 Pesukim dealing with healing and rescuing refer to the healer not to the healed. In addition, Ramban discusses people with illnesses but he did not say that a person drowning in a river should only pray to Hash-m and not cry out for help also from a fellow human-being.

Gut Yomtov

D. Bloom

Here is some further explanation of Rambam's shita relating to the 3 pesukim.

(a) See RAMBAM HILCHOS ROTZE'ACH 1:14 who writes that anyone who could have saved his friend's life but did not do so thereby transgressed Lo Sa'amod. See also SEFER ha'MITZVOS LO SA'ASEH 297 where Rambam explains this further and writes that the Mitzvah not only refers to saving one's friend's life but also to saving his property. See CHOFETZ CHAYIM SHEMIRAS ha'LASHON RECHILUS #9, BE'ER MAYIM CHAYIM #1 DH ve'N'l who learns from Rambam that if one sees someone else who is about to enter a deal with a person who it seems is going to cause him financial or other damage, if one does not inform the relevant party about this, one has transgressed Lo Sa'amod.

(b) Rambam, in MISHNEH TORAH, never mentions the verse ve'Hasheivoso Lo (although he does mention it in Commentary to Mishnah Nedarim as you noted). Minchas Chinuch 237:1 expresses his surprise at this omission but he also refers us to TOSFOS Sanhedrin 73a DH TALMUD. Note 4 to Minchas Chinuch suggests that Minchas Chinuch might be inferring that since Bava Metzia 28b derives a different Halachah from the words ve'Hasheivoso Lo - namely that if one finds an animal one is not obliged to keep it if it costs too much to feed it - it may be that Rambam learnt that one can only use the verse to teach one Halachah and since Rambam GEZEILAH VE'AVEIDAH 13:19 does mention the Halachah about feeding the animal, it follows that the verse ve'Hasheivoso Lo is not redundant to teach that there is a positive Mitzvah to restore someone's body to him.

Even though the aforementioned Tosfos appears to understand that one can learn two Halachos from the same verse, Rambam will maintain that one may not. According to this, Rambam maintains that according to the Halachah, ve'Hasheivoso Lo does not teach us anything about saving life or health because the Halachah follows Bava Metzia 28b and not Sanhedrin 73a.

(c) We suggested before that Rambam does not cite ve'Rapoh Yerapeh because this only teaches that the doctor is permitted to heal but not that he is obligated. However, there is an interesting discussion about what the Gemara in Pesachim 56a reports that King Chizkiyahu hid away the Book of Healing and that the Sages agreed to this. Rambam in his commentary to the Mishnah there explains this at length and seems to compare the right to heal with the right to eat food. CHAZON ISH EMUNAH U'BITACHON 5:5 DH UBE'PESACHIM (this is also printed at end of Chazon Ish TAHAROS) takes issue with this because the Gemara requires a Pasuk - ve'Rapoh Yerapeh - to allow medicine but clearly no Pasuk is required to permit a person to eat, so one sees that the Torah's attitude to healing is different. Further research is required to answer Chazon Ish's question on Rambam.

KOL TUV

D. Bloom

I found a novel explanation in the Rishonim about what the verse ve'Rapoh Yerapeh teaches.

TOSFOS ROSH Berachos 60a cites RABBI YA'AKOV of ORLEANS on what the Gemara states there that from ve'Rapoh Yerapeh we learn that the doctor has permission to heal. R. Ya'akov asks why a verse is required to teach this - why should doctors not be allowed to heal? After all, the Torah states

1) ve'Hasheivoso, from which our Gemara learns that one must restore one's friend's body, and

2) Lo Sa'amod?

R.Yaakov answered that ve'Rapoh Yerapeh teaches that the doctor is permitted to receive payment for his work, because otherwise one would have thought that the doctor is obligated to heal for free.

It seems that R. Yaakov's logic is that since we see from Mishnah Bava Metzia 30b that it is only if one is in the middle of working that one is allowed to receive payment for the Mitzvah of returning someone's property to him, but if one would not lose one's own earnings by performing Hashavas Aveidah, then one is not allowed to receive money for performing this Mitzvah, one might have also thought that a doctor is similarly prohibited to receive payment for performing the Mitzvah of ve'Hasheivoso Lo. The verse ve'Rapoh Yerapeh therefore teaches that the doctor may receive payment for the Mitzvah.

However, see SHULCHAN ARUCH 336:2 that there are limits to the payment the doctor may receive. He may not receive payment for his knowledge, or for his advice to the patient, but may receive payment for the physical effort he expends, or for the time lost when he could have been earning money by some different activity.

KOL TUV

D. Bloom