It's all opinions at the end of the day. On your first paragraph, I think the idea that any other club could have provided Messi with a platform that played to his strengths as effectively as Barcelona (where most of his teammates and coaches came through the same academy as him) has over the last 18 years, is patently daft. That's not a criticism of Messi and it's certainly not a tick for Ronaldo, it's just being realistic. Outside of the very adversarial world of this thread, saying Messi is the best in the world and that Barcelona's set-up gave him a near perfect environment to perform to his best ability wouldn't be particularly controversial or seen as in any way contradictory.
On your second paragraph, again it's my opinion and you're welcome to disagree. If you don't agree with the assertion above, that playing for Barca in La Liga was beneficial to Messi's personal form and suited him more than playing for another club in another league would have done, I wouldn't expect you to agree with the conclusion that he'd have scored fewer goals if he hadn't spent his career there.
To repeat and sort of summarise my thoughts, I think Messi was better than Ronaldo, and that he would have been better than Ronaldo wherever he'd spent his career. However, I don't think Messi would have scored as many goals as he has if he'd spent half of his career away from Barca, and since I know Ronaldo has scored more than him as it stands, it logically follows that I reckon Messi wouldn't be as near Ronaldo in that specific regard were that the case. Luckily for Messi, and his fans, if Ronaldo were to end his career with more goals, it wouldn't magically make Ronaldo the better footballer. Les Ferdinand scored more goals than Cantona, it doesn't mean he was a better footballer. It's probably my fault for coming into this nuance-vacuum of a thread and expecting to be able to say anything about either player that wouldn't get jumped on.