hallel without hodu

A few weeks ago I wrote with respect to hallel on Yom ha’Atzmaut that if you hold that hallel should be said b'toras reading of tehillim as opposed to a formal kri’as hallel that may require a takanas Chazal to institute, then it seems to me that the responsive “hodo” after “yomar na yisrael,” “yomru na beis aharon,” etc. should be omitted.  These responses to the sha”tz (and the doubling of pesukim at the end of hallel) are specific facets of formal kri’as hallel (see Sukkah 38); if someone were just saying tehillim, it would not be done this way. 

Maybe there’s a bit of proof to my point from hallel of Rosh Chodesh, which is just a minhag.  Look at what the Rambam writes (Hil Chanukah 3:8):

 ומדלג ואומר "מה אשיב, לה'" (תהילים קטז,יב), עד "הללו יה" (תהילים קטז,יט); ומדלג ואומר "מן המצר,

Notice: no recitation of hodu at all!

The GR”A (Ma’aseh Rav) had a practice that seems counterintuitive, but actually makes perfect sense now that we know the Rambam.  On Rosh Chodesh, he would listen to the sha”tz say the pesukim of “yomar na…,” “yomru na…” etc., fulfilling his obligation of kri’ah through shome’a k’oneh, exactly like the sugya in Sukkah 38 dictates, and he would just answer “hodu” to the shat"z.  However, on Yamim Tovim he would not rely on shome’a k’oneh and would read all the pesukim.  Why would he follow the minhag as set down in the gemara on Rosh Chodesh, where hallel is just a minhag, and not on Yom Tov?

The reason we don’t rely on shome’a k’oneh like the gemara says is because it’s hard to do -– it’s easy to miss a word.  On Tom Yov, where you need to read everything, the GR"A made sure to say every word.  However, on Rosh Chodesh, where according to the Rambam you can omit all of hodu, the GR"A was not concerned if he missed a word by relying on shome'a k'oneh.