true, though awkward.
also, you have examples such as (from parshat Vayera):
ר"י בר רבי סימון בשם רבי חנין בשם רבי יוחנן העלה אותו למעלה מכיפת הרקיע הדא דהוא א"ל הבט נא השמימה אינו שייך לומר הבט אלא מלמעלה למטה
such a chain of quotations might suggest to me some passage of time, or students of students, though we need to examine the specific personages.

We have:
רבי הונא אמר עקמו עלי את הדרך כדי שלא תהיו נראים באים אצלי
where Rav Huna (if he is indeed the same) was a student of Rav.

We have a statement of בימי רבי חייא בר אבא, implying this was written at a later date. (though perhaps Rabbi Hoshaya wrote this after the passing of Rabbi Chiyya bar Abba?)

We would really need to look at all of the names, and see if they all belong to people contemporary to Rabbi Hoshaya. But why assume that he was the compiler in the first place?

Just a few ideas for how one might explore the pinning down of a specific date.
KT,
Josh

See also here:
http://books.google.com/books?id=OO9wg_PZeNIC&pg=PA148&dq=zunz+midrash&ei=VDk1SaSNMYWcMr6DvKYI#PPA148,M1

A lot of this seems to trace back to Zunz.

Shalom!
Off the cuff:
1) Rabbi Hoshaya Rabba seems not to be the son of Rabbi Yehuda HaNasi, but rather the son of Chama.
http://en.wikipedia.org/wiki/Hoshaiah_Rabbah
though your source says otherwise.
I think it is unlikely that he was the *compiler* of Bereishit Rabba, even though they have a quotation from him at the beginning, which may have caused them to call it by his name. Just as we do not say that Rashi was written by Rabbi Yitzchak, even though Rashi starts with a citation from Rabbi Yitzchak.
Part of what causes me to doubt it is that if it were so that it was compiled by an early 1st generation Amora, then we would expect it to really only contain statements by Tannaim, just like braytot and Mishnayot contain statements by Tannaim. Instead, we have a collection of statements by first and second generation Amoraim included in it, including students of Rabbi Hoshaya Rabba.
Should we have statements by Resh Lakish, or by Rabbi Berechya?
http://www.jewishencyclopedia.com/view.jsp?artid=802&letter=B&search=berechiah

In linguistic terms, the Aramaic found in Bereishit Rabba is the same flavor of Galilean Aramaic found in the Talmud Yerushalmi, such that when I took my grad school course in Galilean Aramaic, we made use of it.

2) Bli neder, I will have to get back to you on the date of other books of midrash rabba. But just because several sefarim eventually received a similar name does not mean that they were compiled by the same individual. My impression is that the dating of these sefarim is *mostly* done on the basis of linguistic grounds (e.g. there may be a different type of Aramaic used in Beresihit Rabba from that of Bemidbar Rabba), stylistic grounds (the types of drash being conducted and preferred), use of source material (thus if it would quote and discuss something clearly later -- I don't know of any such material offhand), and perhaps most importantly, how they are quoting by various Rishonim. Thus, Rashi clearly knows about Bereishit Rabba and quotes it extensively. But other sefarim of Midrash Rabba, not so much.

Thus, while Ramban makes use of Bemidbar Rabba extensively, earlier than that -- Rashi, Aruch, and Yalkut do not make use of it, while they do make use of other midrashim -- strongly suggesting that this came fairly late to the scene. Apparently, they also claim that content-wise (or perhaps stylistically), there is much that can be said to have been drawn from, or connected with, Rav Moshe haDarshan, and so they date it fairly late.
http://www.jewishencyclopedia.com/view.jsp?artid=617&letter=B&search=bemidbar%20rabba

But at any rate, this is what I have heard from more than one of my teachers, in terms of the respective dating of the midrashic works.

KT,
Josh

By the way-- I loved the Ketubah article.

What do I do with my other page? OY!

B"H!! David!! Hi, it has been such a long time!! It is me, Yocheved Hadassah Broscova. I couldn't find my old page, so thought I was erased due to the new construction of this site!! I did in fact start the orphanage in India; it is fantastic. I live in Oregon now with my two "babies ". They are 13 and 14 now. Unbelievable.

B"H". Dear David, Did you see my comment on the SHAMIR? Kol Tuv MICHAEL PLASKOW

B"H! Hope all is well with you! It has been a long time! I am starting an orphanage in India (Mumbai). Do you have friends there? Most of y friends are in Goa.