Your proposal does not really remove the ambiguity as even an adverb at the beginning of a sentence can relate to the whole sentence (imagine a comma after the adverb):
- ofte, labori ne estas facile
Of course there are other ways to make “ofte” relate to “labor-” (rather than to “estas”) but not without changing the structure of the sentence:
- Labori ofte, tio estas ne facila. [To work often, that is not easy.] (note the -a ending, as “facila” now is related to “tio”)
- Ofta laborado ne estas facila. [Frequent working is not easy.]
So you can make it unambiguous but at the cost of changing the structure.
Yes, the infinitive “labori” is used here as a kind of subject, except that the Esperanto grammar prescribes that the following adjective becomes an adverb (facile).
If you want an alternative for the English gerund (“working”) you may use the -ado suffix:
- Laborado ofte ne estas facila
Here facila refers to the noun laborado and so is an adjective. The difference in meaning is, in my humble opinion, negligible. Certainly experts could prove me wrong.
I'm fairly convinced that the infinitive of a verb is a noun in this example. However, it appears to be a just a rule that you use an adverb instead of adjective when the subject is an infinitive (as written in the course notes for Abstract Objects). I'm also interested in learning why this is the case – I don't understand the logic.
- Labori ofte ne estas facile = Working often is not easy
- Ofte labori ne estas facile = Working often is not easy
- Labori ne ofte estas facile = Working is not often easy
- Labori ne estas facile ofte = Working is often not easy
- Ne labori ofte estas facile = Not working is often easy
- Ofte ne labori estas facile = Not working often is easy
In Esperanto, if the subject is a verb, or there is no subject (pluvas, neĝas, etc), the descriptive word modifies a verb, even if that verb has a syntactical function as a noun, and therefore, consistently, the adverbial form is used.
- labori ne estas facila.
- Mia laboro estas facila.
- Erari estas home.
- Neĝas multe, tial estas multa neĝo.