1. IN 'yad ahar eva virajet tad ahar eva pravrajet', the word is 'Vi rajate'. I am not sure how Vai-ragyam comes here.
Almost all translations I have read, translates them as 'lack of rajas'. I see Rajas as energy. In fact many translations also call Rajas as energy in this case. Some others translate rajas as atomic, minute, dust and hence vi-rajas as lack of dust and hence pure, clean etc. and then say one who is pure can start wandering. But that's not what I see.
We have to read it fully. There is a rule that Yajnavalkya mentions. There is an exception. The exception is for the Virajas, who lost energy.
If can you give me the verse by which it connect to vairagya, it would help. Thanks for it.
2. I translate 'utsann agnir' as one who has abandoned the agni rituals and 'anagni' as one who never followed those rituals. I did not translate them as lack of energy. I translated it exactly like you.
3. Yajnavalklya simply says not just brahmacharya, but grhasta, vani, vrata snAtaka (who completed school, but not earned his degree), vrati (who follows all the vows and rituals, but probably never went to school/gurukul), utsann agni (who abandoned his rituals), anAgni (who never did it), if that person is Virajas /lacks energy (vi-rajate) can start saMnyAsa.
That is the exception part.
-TBT