1) Assuming team payrolls are basically even across leagues (excluding the Yankees who are an outlier), the AL team is at a disadvantage. Let's say bothLet me expand on this point. Let's assume that on average NL and AL teams have about the same payrolls (interestingly if one takes the Yankees out of the equation the average payrolls are almost exactly the same -- $74,923,614.3 for the AL and $74,954,351.25 for the NL). Of course even if the salaries were not equal, that would not be an argument against interleague play but against salary inequity, which is a problem in intraleague play as well.
the AL and NL team has 100 dollars to spend. If the AL team has a DH they will be forced to spend x number of dollars on the player who is playing the DH's natural position (Player Y). The NL team can spend the x dollars on other positions. So while the AL team will have a good hitter on the bench in NL parks (because Player Y won't start), the NL bench and bullpen (or other positions) will be better because they can spread that money out on the other positions. And when there's no DH, it's more important to have a deep bench and bullpen, because if it wasn't, NL teams would just spend x dollars on one slugger to keep on the bench. So the NL team will have the advantage here.
Let's use a video game metaphor. Assume two leagues, each league containing teams that have a 25 man roster. All the teams have 25 players who are exactly average (and therefore exactly the same). Each team is given 100 points from which it can add to the players. So, for example, a team might add 10 points to their first baseman, which would give them an excellent first baseman, but they would only be left with 90 points to spread around the rest of the team.
Depending on the league rules, the teams will allocate the points in different ways. So if one league allows its second basemen to have four strikes, each team in that league will probably allocate more points to their second basemen than the teams in the other league. In real life, one league allows the DH and the other does not. So we would assume that the teams in the AL, which allows the DH would allocate more points to a 9th hitter than in the NL.
The NL team, it would be expected, would spread out the points among the other players. But when it comes down to it, the NL and AL teams are exactly even, although their talent is spread differently among the players.
If we follow the rules of interleague play, when the teams from different leagues play each other, the away team will always be at a disadvantage. When the NL team plays in the AL stadium, the rules are tilted in favor of the team that allocated more points for a ninth hitter. When the NL team is home, it has an advantage because the AL team is constructed for a league where a ninth hitter is meaningful; in the NL having an above average ninth hitter is not helpful if it means the rest of the team will be worse off (because if that was the case, then the NL teams would just spend as much points on the ninth hitter as the AL teams).
In other words, assuming everything else is equal (and if everything else is not equal that’s not an argument against interleague play but for fixing the inequalities), interleague play does not create disadvantages for either league as long as both leagues have an equal number of home games.