Sky was buying shares in clubs and leveraging influence over them waaay before 1998. Mainly in order to secure PL TV rights. Sky bought loads more shares in Man Utd than any of the others though, both in %age and value (more than needed to do with any of the others!) and Man Utd were the only club that they ever tried to buy outright. Granada's investment in Liverpool and Arsenal was more to do with them trying to exploit online and overseas, was a different cycle in terms of media rights and is incomparable as it never yielded any advantage. It was a move made by Granada when it looked like overseas TV rights could be split from the communal PL pot as it is in La Liga, which it never was.
The whole if your auntie had balls argument is neither here nor there. The PL clearly saw a conflict of interest with Sky owning as much of Man Utd as they did as Sky were forced to reduce their shareholding and the Monopolies Commission also clearly saw a conflict as the they refused to ratify Sky's takeover. Proof is in the pudding as they say.
ManU floated in 1991 so the stadium could be renovated, apparently only selling half the shares available. Sky bought United shares in 1998, they received investment just like Liverpool and all the other teams from Granada/Sky/NTL etc. Again we are talking about 1998 onwards, so UTD had no advantage over other teams at the PL launch.
Sky secured the PL rights at launch after outbidding LWT(who were instrumental in setting the break away league in motion after meeting with Utd, Liverpool, Spurs, Arsenal and Everton) so not sure why you single out UTD.
Sky were ordered to reduce shares in late 1999 after the blocked takeover not before. NTL's takeover of Newcastle was also blocked. The Sky takeover bid has nothing to do with ManU winning the league in the 90's though so not sure why it gets mentioned.
Utd won 4 out of 6 titles between 92-99, they were terrible in Europe until the treble... that was still dominated by European teams, mainly Italy. They were successful, but its not like they won everything in sight, its like some weird obsession from other fans that they were given it on a plate.
Not sure what the auntie balls bit is about.