In Mat 6:10 "as" could be "because", alternatively it could be "in the same way". Your will be done on earth because it is done in heaven, or, your will be done in the same way/as it is in heaven? What are the supporting arguments for the best choice?
The following translations have rendered Matthew 6:10
ως εν ουρανω και επι της γης [Elzevir 1624]
as follows :-
Tyndale : as well in earth as it is in heaven
J N Darby : as in heaven, so upon the earth
YLT: as in heaven, also on the earth
KJV: as it is in heaven
The KJV is, as you point out, ambiguous, but the other three are not and those three translate the phrase in the manner of 'like the way it is done in heaven' not 'because it is done in heaven'.
Young's Ananlytical Concordance lists the following Greek words as those which have been translated 'because' in the KJV :
δια 29 times; εκ twice; εν 3 times; επι once; προς once; χαριν twice.
ως has never been translated 'because' in the KJV.
It has been translated :- as 344, about 14, as it were 20, how 19, when 41, while 4, as soon as 7, even as 6, that 5 and sundry others. But never 'because'.
Translating ὡς because is very rare. The Hebrew word כִּ֣י is the word that means as, because. See the following. The graphs from Bible Word Study Guide in Logos Bible Software 7 indicate the number of times the word is translated a particular way in the designated version. The usage of ὡς across the New Testament shows the translation of "as" with the same meaning as "like" fits the most likely meaning. However, that meaning in this context is essentially the same as "in the same way." The most likely Greek word to mean "the same way" is οὕτως, the adverbial form of the word meaning "this." Key to the meaning is the Greek word καί, which is usually translated "and," but in this context means "also." Thus, a literal translation of ὡς ἐν οὐρανῷ καὶ ἐπὶ γῆς is "as in heaven, also on earth."
Here's how the Greek word ὡς is translated in the ESV:
Here is how the Greek word ὡς is translated in the KJV:
Here's the Hebrew words translated with ὡς in the LXX:
Here the Greek words translated because in the ESV:
I would certainly translate the word ὡς (hos) as "like" or "similar to". Thus, to labour the point somewhat, it would be possible to translate this phrase in the Lord's prayer as:
Let your will come to be in the same way in heaven and also upon the earth...
Or in better English
Let your will come to be upon the earth in the same way as it is in heaven...