There are a lot of publishers accepting unsolicited novellas right now.There are a lot of shoulds in contracts, but often small publishers don't know, don't care, or won't bend.
Last week, there was a pretty public brouhaha on Twitter. A publisher had ‘accepted’ a number of novellas earlier in the year, held the rights for about 6-8 months, then killed all the deals, returning the rights to the writers. The contracts had no advances and no kill fees (which is normal for short stories).
I understand short stories having simple contracts but shouldn’t novellas have contracts similar to novels?
Also, would an agent be interested in an unsigned writer who has an offer in hand for a novella (I’d expect not)?
Standard language in a book contract requires publication within a certain amount of time (18-24 months) or the deal is off and the author keeps any money paid to them.
Of course, with no advance, there's no money to keep.
Honestly you're better off with a company that returned the rights instead of publishing and never paying you.
My guess is the publisher over-bought and came to their senses before any real damage was done.
Real damage is screwing up any chance of reselling the novella.
Whether or not an agent is interested in a novella with an offer in hand is too individual to answer here.