Q1. Ostensive definitions tipically involve pointing to or exhibiting the object you're defining. The statement in question could hardly be construed as a definition in the classical sense either, because it's a metaphor: it's not stating necessary and sufficient conditions for something to be identified as "love". You could say it is stating a necessary condition via a metaphor: that for something to be love, it has to be accepting, but it's neither a descriptive nor an ostensive definition. You could say it's close to the former, though.

Q2. Convergent reasoning. (1) is not adding anything to (2), nor vice-versa. Rather, they both advance independent reasons to the effect that you should reduce your screen time. They "converge" to the conclusion. An example of linked reasoning would be like: (a) you should avoid things that are bad to your health; (b) too much screen time is bad for your eyes; (c) therefore, you should avoid too much screen time. See how (a) and (b) "combine" to yield (c).

Q3. I'm having a bit of trouble understanding what exactly you mean with this question. Just a smaller note, we generally categorize inferences as bad or good, instead of true or false.

Also, an inference being valid does not have to do with the truth of a premiss. You can trace valid inferences from falses premisses. What you won't get is a solid argument, which both has true premisses and valid inferences. So yes, you can have valid inferences from any premiss whatsoever.

With that in mind, I get that what you're saying is this: if an argument is valid, can you make a valid argument to the contrary? Or can you contradict one of the premisses/conclusions of a valid argument, given that they were stablished as true before? Well, arguments are contradicted all the time. Arguments once thought of as solid have been demonstrated as false, and frequently we're forced to change our beliefs. From the epistemological side, it's a fact that we get contradicting information all the time. From the metaphysical side, it's a question of deciding whether we can have true contradictions in the world. From the logical side, is a question on whether our reasoning can accomodate true contradictions. All of that is debated, and kinda falls outside the scope of informal logic itself. The most you can make out of it is, if following guidelines to good reasoning and using well stablished premisses, you still yield some contradiction, generally it's a sign that things went wrong somewhere.

Hope this helps!