To my ear, (1b) sounds awkward. I’m writing this mostly to try to make sense of why it sounds awkward for myself.
(1) a. 芬蘭有很多湖。
This DigMandarin article on existential sentences simply asserts that “在” cannot be used like this. However, I think there is a more nuanced explanation. C.-T. James Huang, in "Existential Sentences in Chinese and (In)definiteness", claims that all existential sentences in Chinese have this form.
（NP）… V … NP …（XP）
The first position can be occupied by a locative noun phrase like “這裡” or by a noun phrase representing a possessor, but in either case, there is an optional subject at the start of the sentence.
Theoretically, there can be a locative prepositional phrase at any position in the sentence.
This still sounds a little awkward to me. But when I think about moving the prepositional phrase to the beginning of the sentence, I feel a strong need to insert a subject.
On the other hand, I don’t feel the same need in this sentence.
What I believe happens when I hear (1b) is that, after I hear the prepositional phrase “在芬蘭”, I’m anticipating a noun phrase that will serve as the subject of the sentence. There is none, so I assume that the sentence has an implied subject. At the end of the sentence, I’m left wondering, “who/what has a lot of lakes in Finland?”
The difference in (4) is that, although there is no stated subject, it is easy to imagine the implied subject being the speaker. “On the table, (I) have a lot of books.”
Huang discusses how there’s an interaction between whether an existential sentence has a subject or not and the “definiteness” of the object that’s allowed. Typically, the object of an existential sentence needs to be indefinite, but in existential 有-sentences that have subjects, “syntactically definite but semantically indefinite” objects are allowed.
I believe something like this is what’s going on here. Huang says that “definites” include “NPs with quantifiers like ‘most.’” If “很多湖” is definite, then a sentence like (1a) must have a subject, so we are not allowed to use the prepositional phrase “在芬蘭” instead of the noun phrase “芬蘭”.
Huang also discusses a type of existential sentence where the verb is a “locational verb.” These sentences must always have locative noun phrases in the subject position, and they cannot be prepositional phrases. The exception is if there is an implied agent. For example, (5a) and (5b) are acceptable, but (5c) is not.
(5) a. 墙上挂着一顶帽子。
I think this is perhaps not very related to what’s going on with (1b), but it just goes to show that there’s no simple rule for whether existential sentences with 在 are well-formed or not.
Lastly, in my view, the existential use of “有” functions very similarly to the English "to have," except that the subject is optional in Chinese but required in English. Because of this, when there isn't a subject, English switches to "there are." The parallel is clearer in languages like French, where "avoir" (to have) can be an impersonal verb. Here's an example.
(6) a. 有 人。
b. Il y a des gens.
it there has some people
Here are my translations of the sentences you gave as examples of when "to have" doesn't work well, and hopefully they're helpful for you.
(7) a. 桌子上有很多書。
'The place on the table has a lot of books.'
'The place inside the drawer has a lot of paper.'
'This place has a lot of people.'