I wouldn't use
were. The first sentence has
was, a
past tense.
could have tipped also indicates the
past.
if there were is a hypothetical
present, so it doesn't fit with the past tenses.
(Note that in hypothetical clauses, generally introduced by
if,
what is usually a past tense is treated as a hypothetical present, and
what is usually a past perfect tense is treated as a hypothetical past.)
CJ