The answer to this question is evident when you practice reading the MT correctly with all of the readers marks, or at least listen carefully to a reader who knows how to vocalize correctly. I have been blessed to dwell amongst the returnees of Habban who faithfully preserved such Hebrew reading skills.
There is no text-critical problem in the use of שדי מואב and שדה מואב in the MT of Ruth. The difference in spelling reflects the requirements of the vocalization.
The argument for a textual problem arising either from scribal error or from elision source material is difficult to make considering the number of times this distinction occurs in Ruth, and in particular that it occurs together in a single verse, 1:6.
In both usages in Ruth, שדי מואב and שדה מואב the form is singular, not plural. The plural form would be שדות מואב as in Nehemiah 12:29 (MT):
ומבית הגלגל ומשדות גבע
and other verses. Although both שדי and שדה in construct form (סמיכות) are singular in meaning in Hebrew (both OT and modern), in western languages they need to be translated as plural in order to avoid an implied definite article, "[the] field of Moav" in western languages, which would leave the reader asking, which particular field the verse intends.
The usage שדי in meaning "field of" in compound form (סמיכות) is common in the MT:
- Isaiah 32:12 - על שדי חמד על גפן פריה
- II Samuel 1:21 - הרי גלבוע ... ושדי תרומות
- Psalms 132:6 - מצאנוה בשדי יער
- II Chronicles 31:19 - בשדי מגרש עריהם
- Proverbs 23:10 - ובשדי יתומים אל תבא
- Nehemiah 12:44 - לכנוס בהם לשדי הערים
In Ruth, as in most other instances, שדי in compound form is a contraction of שדה. When the position of the compound in the verse requires contraction, the vocalization of the consonant ה is dropped and only it's vowel (תנועה), represented by י, remains. The letter י is used here not as a consonant but as a reading aid (אם קריאה). It's like a really short ה.
Ruth 1:1 - בשדי מואב is vocalized with a shofar holech under the ד, a dependent intonation that leads to the following intonation, a zakef katon over over the א of מואב. This is followed by a dependent clause. In this situation, the zakef katon is shortened ("קצרה") and the intonation is falling, like a shortened atnahtah. שדה must then be contracted to שדי to allow the emphasis to be on מואב.
Ruth 1:2 - ויבאו שדי מואב has a contraction mark between שדי and מואב in the printed editions that include the vocalization. (The contraction mark is not required in Ruth 1:1 because of the ב preceding שדי. That additional letter is a hint in itself that a contracted vocalization is called for.) The vocalization under the א of מואב is tifha, a falling tone similar to a shortened atnahtah. The compound form is at the end of a clause, before "and they resided there".
Ruth 1:6 - משדי מואב is at the end of a clause marked with an atnahtah under מואב and followed by an independent clause. This is equivalent to being at the end of a verse, so שדה must be contracted to שדי and the vocalization of מואב drawn out, as it would be if it were at the end of a verse.
בשדה מואב is at the beginning of the second, independent, clause of the verse and is vocalized with shofar holech under the ד and a zakef katon over the א. This is a long, rising tone (מלך) that indicates that the entire compound should be pronounced slowly. So there is time to fully vocalize the ה.
Ruth 1:22 - משדי מואב is again at the end of a clause marked by an atnahtah and followed by an independent clause, so שדה is contracted to שדי to allow מואב to be drawn out.
Ruth 4:3 - משדה is marked with a maarachah, literally an "expander", under the ד. This is a rising tone before the end of the verse that indicates that the vocalization should be drawn out, so there is time for a fully vocalized ה.