Auspiciousness and strength are two different things. Mercury is too weak in shad bala. being vargottama and being included in parivartana It's auspiciousness is increased. But It still lacks the drigbal and sthanbal it need to exert it's effects.
And for parivartan yoga, we usually think that it erases all afflictions, but this is not so. in ancient texts planets included in exchange should be considered as conjunction. In this case both planets share a free flow of energy between them. Hence their effect is combined resultant.
Another fact is that, when a planet is carrying both malefic and benefic properties (Which most of the time every planet does) then it will exert both of it's properties and it does not get cancelled. So a person gets both malefic and benefic results from the planet period depending upon the what percent of it carries benefic and malefic properties respectively.
hence in this chart, Budha has although become auspicious but being weak it is still unable to give full performance. Vargottama planet is supposed to give good results but it need enough energy to transmit those results. Being vargottma does not cancel it's badhak properties and also can not hide it's weakness.
if you look at the palm of the native you will find that the mercury mount is also not that stron although the mercury line may be in good position.
Dear friend, As I told I am just a learner of astrology, I will be thankful If you correct my mistakes
Kind regards
Atul