IMO, I really think one needs to be careful with these blanket statements. Obviously if a kid came to the US from Mexico the summer after the 8th grade and then went to a HS in the US, then s/he should obviously study a different language in HS. However, if the student was born in the US to parents who came from another country, I think the answer needs to be “It depends.” Being a heritage learner covers a very wide continuum. Being Latin@ does not always mean proficiency. As an example, listen to JLo speaking Spanish. The student may understand Spanish, but not speak it. Or s/he may speak it, although not always grammatically correctly, but cannot read/write it. Or s/he may be fully proficient speaking grammatically correctly on an informal level, but cannot communicate in a formal situation. So in many of these instances, it may be advantageous for a students to gain closer to proficiency in the heritage language first. Just because Spanish is spoken in the home does not mean that the student does not need formal instruction in the language. If that were the case, US students would not need to take 12 years of English.
Now Asian heritage learners may be a different case, since many attend Saturday Chinese (or Korean. etc. ) school from a young age, so it might me more advantageous to pick a different language in HS, since they’ve received some formal instruction along the way. In all instances, however, I think we need to give the HS’s some credit for knowing what they are doing. At my HS, if you scored too high on the placement test, you were strongly advised to pick a different language to study. Now all HS’s may not be that diligent, and some students may be gaming the system for the easy A.
In terms of the Subject Tests, you can follow @ucbalumnus ’ advice above - feel free to submit it as an N+1. Personally, I think this is a waste of time and money, since the Subject Test tests at a relatively low level, but YMMV. I have no issue with heritage speakers taking an AP exam for college credit. However, AO’s are not going to care if you submit an AP score, since AP scores really don’t mean much in the admissions process; they are primarily used for credit and/or placement.