The gemara on 2b discusses why in the mishna we allow for watering a Beis ha'Shelachin field during shemittah. The gemarah gives an answer from Rava that in regards to shemittah only the four prohibitions expressly listed in the torah relating to shemittah of ketzirah, zeriyah, zemirah, betzirah are assur. And therefore although watering will have the effect of having the crops grow, nevertheless since this is not the normal act of zeriyah (which is the act of dropping seeds in the ground and covering them), it is therefore permitted during shemittah in a Beis ha'Shelachin. The gemarah then asks that a braiysa assurs many more things on shemittah than just those four mentioned above including iddur (hoeing) and nichush (trimming bad grass to allow good grass to grow), etc. To this Rava answers that only the four mentioned in the torah are biblically prohibited, whereas the things mentioned in the braiysa are only an asmachta.
Rashi (3a) asks that there is a stirah by iddur in the braiysa that in one place in the braiysa it prohibits it and one place it permits it. To this, Rashi answers that just like we see a distinction on 4b between ugiyos (areas of water collection surrounding plants) on choel moed between whether the soil has been worked or not (chadati versus atiki) so too here that by chadati it is prohibited by iddur and only by fixing up atiki is it permitted. The Sfas Emes asks on Rashi that that later gemarah which rashi uses to answer his question is talking about moed where tircha is assur, as opposed to by shmittah which is the discussion of the braiysah on 3a and where tirchah is not assur?!? The Rashash has a much more logical answer that the iddur that is assur is where it is for a revach as opposed to iddur that is to prevent a hefsed, that this type of iddur would be permitted. The Rashash's answer fits very well with the flow of the gemarah on 3a and how the gemarah dealt with a similar contradiction in the braiysa relating to kishkush.
The Ritva asks a question that why should there have been a differentiation between nichush and watering a field that watering the field is muttar during shmittah (as we see in the mishna on 2a) and nichush is assur (as we see in the braiysa on 3b), since earlier on 2b there was a machlokes between Rabba and Rav Yosef about watering fields and Nichush, and in the gemarah, the two acts are lumped together. Therefore if we permit one, we should permit both, or vice versa? The Ritva answers that the braiysa that prohibits Nichush is talking about by a Beis ha'Ba'al and not a Beis ha'Shelachin and therefore, by a Beis ha'Shelachin nichush would be permitted similar to watering the field, as can be seen from the mishnah itself.
On 4b the gemarah discusses the opinion of Rabbi Elazar Ben Azariah as to why one cannot dig an amas hamayim initially on Moed and Shmittah. And the Gemarah says that the reason why on Shemittah it is not allowed, despite tirchah not being prohibited on Shmittah is because according to one opinion, it looks like Iddur (hoeing), and is assur as a maaris ayin. The problem is: 1) that based on the Ritva's answer, iddur should be absolutely permitted in a Beis ha'Shelachin on Shemittah. If so, what kind of maaris ayin concern do we have to justify prohibiting digging the amas hamayim on Moed - particularly when the simple understanding of the statement of Rabbi Elazar ben Azarya is that we are talking about a Beis ha'Shelachin, since a Beis ha'Ba'al would not need water diverted to it with the same urgency. And 2) Why anyways should we prohibit based on maaris ayin, since after all there is a legitimate way that iddur is permitted as we stated above that there was a stirah in the braiysa on 3a, and why should the onlooker assume the worst, if very legitimate reasons for having done iddur may exist?
Shmuel Gertel, Baltimore, MD USA
I have not been able yet to look much in the Mefarshim, but I will make some initial comments to suggest approaches to tackling these questions.
1) I think the central point of your comments is possibly the opinion cited by the Ritva that differentiates between Beis ha'Ba'al and Beis ha'Shelachin concerning Nichush. In my opinion, it is important to realize that this opinion is a very big Chidush and, quite possibly, this is why the Ritva writes immediately that this opinion does not appear to him to be correct. I will try to clarify.
2) When we learn the first Mishnah in Moed Katan, we see that there is a special leniency relating to watering the Beis ha'Shelachin on Chol Hamoed. Rashi explains that there is a great loss involved if one does not water it. This shows us that a small loss would not be a sufficient reason to permit the watering, but this is a special Heter because of significant financial distress.
3) The simple way of understanding the Mishnah would seem to be that it is only watering which is permitted for the Beis ha'Shelachin. Since the Beis ha'Shelachin is a field in a specific geographical location, as result of which if it is not watered it is in danger of drying up entirely, Chazal gave a special Heter to water it. However, according to the simple understanding, other Melachos are not permitted for the Beis ha'Shelachin.
4) However, the Ritva makes an inference from the Gemara on 2b, which mentions Mashkeh and Nichush together. On the basis of this inference he asks why Mashkeh should be permitted in the Mishnah while Nichush is prohibited in the Beraisa. To answer this contradiction, his first answer says that the distinction between Beis ha'Ba'al and Beis ha'Shelachin applies also for Nichush.
5) This in itself is a big Chidush, but we should be careful not to take the Chidush even further. So it is only Nichush which one could possibly concieve is permitted for Beis ha'Shelachin, because it is mentioned in the Gemara alongside Mashkeh, but no other Melachah receives this special Heter. Therefore, even the first answer in the Ritva would not permit Idur on Chol ha'Mo'ed or during Shemitah in a Beis ha'Shelachin. One would not suffer a great loss if one does not do Idur in the Beis ha'Shelachin, and this is the only reason why Rashi in the Mishnah (2a) permits watering in the Beis ha'Shelachin.
6) This is not similar to the distinction made by the Gemara on 4b between new and old ditches. Rashi there (4b, DH Ha) writes that the reason why it is permitted to re-dig old ditches is that it does not involve hard work. Similarly, Rashi on 3a (DH Lo Yekashkesh) writes that Idur is permitted in a place which has already been dug up. This logic does not apply when we try to make a distinction between Beis ha'Shelachin and Beis ha'Ba'al.
7) This, then, is an answer to your first question: There is no reason to permit Idur in a Beis ha'Shelachin, where the crucial factor is quite different, namely could a significant loss occur.
B'Ezer Hash-m, I will address your second question in a further reply.
Kol Tuv,
Dovid Bloom
Now to answer your second question:
1) The basic idea behind the prohibition against Mar'is ha'Ayin is that even though the action being performed might be totally innocent, still a person is obligated to act in such a way that he will not be suspected by others of wrong behavior, even though in reality he is doing nothing wrong.
2) We see this in one of the major sources for the idea of Mar'is ha'Ayin, in Beitzah 9a. There the Gemara discusses one whose clothes became wet when he was walking in the rain on Shabbos. When he reaches home he may not hang his clothes out to dry on Shabbos because people might think that he laundered them on Shabbos. We learn from this that even though this person did nothing wrong at all, and he had no intention at all to launder his clothes, he still must be careful that people not suspect him of doing this.
Kol Tuv,
Dovid Bloom