予備 and 代わり: a prepared spare and actual substitution
Compare spare batteries with using soy milk instead of milk. Having a spare does not by itself establish that it has been used as a substitute.
Have a spare for a possible need
予備, read よび, means spare or reserve. 旅行には予備の電池を二本持っていく。 says that someone takes or will take two spare batteries on a trip.
Two is the number of spare batteries to take. Prepared spares are not already used replacements, and the nonpast sentence does not report completed past use.
Read the item actually used instead
代わり, read かわり, means instead or substitute. 牛乳の代わりとして豆乳を使った。 says that soy milk was used instead of milk.
The substitute and what it replaces are both supplied. A prepared spare can become a substitute, but preparation and actual substitution are different relationships.
Leave a spare and act in another person's place
予備の鍵を家族に預けておいた。 says that someone left a spare key with the family. Leaving the key does not report that it has already been used.
忙しい父の代わりに私が荷物を受け取った。 says that the speaker received the package in place of the busy father. Read the replaced person and the receiving actor without limiting 代わり to objects.
Does preparing 予備の電池 alone establish that the batteries have already been used 代わりとして?
Show the answer and why
No. Preparing spares and actually using them as substitutes are different relationships. Use needs a separate action statement.