We know that whenever we have a Bousfield localization between two simplicial model categories, this gives rise to a reflective subcategory in ∞-categories (or coreflective, depending on the direction of the Bousfield localization).
I’m interested in what happens if the model categories at issues are not simplicial, or even in the intermediate case when the categories themselves are simplicial, but we don’t know that the functors that comprise the Bousfield localization are simplicial. Does this still give rise to a reflection of ∞-categories? At least, when the model categories are combinatorial?
Related question: we know by a result by Dugger that every combinatorial model category is Quillen equivalent to a combinatorial simplicial model category. Is this assignation functorial? Do functors and adjunctions between combinatorial model categories get promoted to simplicial functors and adjunctions?
This is not a full answer, but too long for a comment.
Here‘s a relevant paper. Mazel-Gee proves the folklore claim that a Quillen adjunction induces an adjunction on underlying ∞-categories with very little assumption on the model categories involved.
I think it’s proved (or mentioned) in the appendix (at least) that upon passing to homotopy categories, the co/unit is the same as the one in the derived adjunction introduced by Quillen, in particular, in the case of a Bousfield localization, the (co)unit is an equivalence, and so it is an equivalence also at the level of ∞-categories (C→ho(C) is conservative), and so it also induces a reflective subcategory.
In other words, I think you don’t need properness or anything like that.
For your question 2, I’m not sure about functoriality per se, but Quillen adjunctions definitely induce Quillen adjunctions: that’s because Dugger’s universal homotopy theory satisfies some almost universal property.
You can have a look e.g. at propositions 2.3, 5.10 and theorem 6.3 in Dugger’s paper : suppose M⇄ is a Quillen adjunction, then by 6.3 you can make it into a Quillen adjunction UC/S \rightleftarrows N for some C and S and then with 5.10 you can lift it to UC/S \rightleftarrows UD/T
To ask whether it can be made simplicial is to ask whether 2.3 can be made simplicial, i.e. suppose given \gamma : C\to M where M is a simplicial model category, can the functor Re: UC\to M from 2.3 be made simplicial ?
Now I’m not entirely sure the answer is yes, but I would guess that it is if M is nice enough. At least, Lurie seems to indicate something similar in the last sentence of the proof of A.3.7.6. from Higher topos theory : “The proof given in  [ Dugger’s paper ] shows that when \mathbf A is a simplicial model category , then F and G can be chosen to be simplicial functors” – the context is not quite the same, so that is not claimed in HTT, but something similar is.
Hopefully someone can comment on whether this is the case (that’s why this answer is not a complete answer)