Why konsistanta instead of konsistas?
organo konsistanta el karnaj fibroj kaj servanta por movi la diversajn partojn de la korpo
is ReVo's definition of "muskol/o". (Also reflected here in Simpla Vortaro.)
For dictionary definitions of words or terms, it's not unusual to not be complete sentences and to lack a conjugated verb. This is done in English ...
ŝanĝi: alter or replace
"ŝanĝi" is to change <someone or something> (i.e. to alter <someone or something> or even to replace <someone or something>. It's a transitive verb, which means that it always requires a direct object: the thing or person being replaced or altered.
Ŝi ŝanĝas la oleo. — She changes (replaces) the oil.
There are two main clauses here, "li pensis" and "ĝi resonis", that are made parallel, equal-valued (sorry, I know the term neither in English nor in Esperanto) by the word sed. At least to me the example appears like, that the actions in the main clauses are taking place at the same time, thus I would choose resonis.
esti leg(a/i/o)nta is possible in infinitive. See this sentence from the "Ekzercaro de la Fundamento de Esperanto", mainly the part between parenthesis:
... ke tio, kion mi diris, estu vera (aŭ mi volas esti dirinta la veron).
It might also come up when you really want to emphasize the time, in sentences like:
Sen esti leginta la enhavon, mi povas aserti ...
Plena manlibro de Esperanta gramatiko (PMEG) doesn't mention such forms at all. It takes up only A-, E- and O-forms of participles. While not officially being as high authority as Akademio de Esperanto, PMEG is for all practical purposes "the grammar book". It has lot of examples and since there are none involving any participle form with any verb ending, I ...
The accusative case is used in Esperanto for a couple of things. The most obvious one is the direct object. Another one is to show movement to a direction. Thus you can say
Mi veturas Varsovion.
Ni iris la urbon Bostono.
However many considers this way old-fashioned and contemporary way is to use a preposition
Mi veturas al Varsovio.
Ni iris ...