The modern concept of faith makes no more sense in Thedas than atheism does. Pre-modern people did not have 'belief without evidence;' an educated medieval man could provide both philosophical and scientific arguments for the existence of God.
For example, he could point to instances of holy people praying over people with horrible sickness and those afflicted being miraculously healed. He could provide the testimony of other men of good standing and sound mind who witnessed the event. He himself might have visited the body of a saint who died over a century ago and found the corpse whole and giving off a pleasant scent.
When asked to provide non-divine reasons for this being possible, the best you could say is 'maybe the dude who was coughing blood and covered with pox wasn't really that sick.' You can't point to medical journals that show prayer doesn't help. You can't talk about placebo effects because that concept doesn't exist. You have no knowledge of pathogens and immune systems. You probably don't know if the illness the sick man had is one that tends to have a quick recovery time or not -- you probably don't even know exactly what illness he has as there are dozens lumped together as pox.
Any argument you could mount against this miraculous healing would be without evidence.
Your example is valid, but I contest its applicability to the problem.
You say it would be appropriate to adopt a "medieval" mindset in this. There are several problems with this statement:
(1) Why should I do what the writers clearly don't? Thedas is, especially with regard to the philosophy of ideas, most unlike the European middle ages. In general, I see the prevalent mindset being more of a late Renaissance type with a smattering of Enlightenment-period humanism added. Most notably, both Aldenon and Anders justify their pro-mage stance with a decidedly modern concept of human rights which had its beginnings in the Renaissance and the Enlightenment but didn't exist in the European medieval period. Also, Avelline says she doesn't find the evidence for the Maker convincing, and Morrigan says she doesn't believe in any higher being, so that's clearly a mindset we're supposed to accept as plausible for an at least medium-educated Thedosian.
(2) Divine intervention is an implausible explanation. That doesn't mean it can't happen, but it does mean we should exhaust the plausible explanations first before going there (BTW, critical examination of miracle claims was actually done by the Church from the late middle ages on). That consideration applies to all "adopted mindsets" since knowledge of some things was unknown in some time periods, but people were not stupid. In general, healing properties of certain substances were well known throughout history, and while I haven't studied history, I think that the concept of poisons and antidotes was rather well known at least among the more educated people of most time periods since antiquity.
Then add magic, and there are several more plausible avenues for the examination of a "miraculous" healing than divine intervention. Divine intervention only rises to the top of the "plausibility hierarchy" in some people's minds because of the identity of the person involved, which usually doesn't survive the burning of the dead body (as the qunari would say so that, too, is a documented way of thinking), combined with a pre-existing disposition towards belief that, as explained above, is not a necessary part of the Thedosian mindset.
(3) The question asked was "is there evidence for the Maker". I took that to mean "Are there things in the lore which indicate that we, as the players, are supposed to see the Maker as real." To that, the answer is clearly "no", well in line with the developers' stated goal of leaving this a matter of interpretation. "Yes" is the answer to the question "Is itt possible that the Maker exists?", but that doesn't require evidence but only a consistent narrative free of contradictions with observable reality, which Andrasteanism does provide.
These two questions are distinct from "Is there evidence for the Maker that an average Thedosian would accept". The answer is possibly yes, but in a world where magic is consistently applied to everyday problems (see Anders in Kirkwall) people would be far more likely to suspect magic than divine intervention. Yet again, this is only changed by a pre-existing bias towards (a specific) belief, but even then "It's a kind of magic" suggests itself strongly as an alternative. Now ask the question: "Is there evidence for the Maker a highly educated Thedosian would accept?" I don't know the answer, but even if is it "yes", Andraste's ashes would not count as such, because that educated Thedosian would know that "It's a kind of magic" suggests itself much more strongly as an explanation.
(4) Even were I to adopt a renaissance mindset towards healing, I would try to explain things in terms of humors and essences as was common at the time, instead of considering divine intervention as the first hypothesis. The main reason why divine intervention was widely accepted because the existence of the creator god was presupposed, and as I said, if you do presuppose it, divine intervention becomes more plausible as an explanation. We are, however, neither required nor called to pre-suppose it, as Avelline's and Morrigan's examples show.
What this means is there is enough precedent and knowledge to dismiss everything presented so far as evidence for the Maker as irrelevant, both in-world and out-of-world. The existence of the Maker has to be considered possible but unsupported by more than circumstantial evidence.
As an aside, btw, I reject the statement that we should adopt the mindset of a historical period of which a fantasy story is reminiscent. I reject it because the story is set in a fantasy world, not in history. That it shares some elements with historical times and places does not mean it has to share other elements. Also, the story is written by 21st century people for a 21st century audience and it makes no claim to accuracy even in those elements reminiscent of historical times and places. It derives its meaning, if any, from how it comes across to us 21st century players and readers. I experience the story as a person of my time, and roleplay characters with mindsets I find appropriate based on the lore and my own preferences - any plausible Thedosian mindset is acceptable, and that includes critical thinking, standards of scholarship and not seeing the divine in everything.