There are a number of questions raised here, so I'll take them one by one.
First, was your 2C bid generally alertable? Yes, an artificial and forcing 2C bid is alertable according to the ACBL Alert Procedures. In the section on artificial bids, it is not listed among the "Other Artificial Bids" that should not be alerted.
Second, was the penalty appropriate? No, by my reading of the Laws of Duplicate Bridge. Law 21 (Misinformation), Section B (Call Based on Misinformation from an Opponent), reads in part:
- (a) Until the end of the auction period (see Law 17D) and provided that his partner has not subsequently called, a player may change a call without other rectification for his side when the Director judges that the decision to make the call could well have been influenced by information given to the player by an opponent. Failure to alert promptly where an alert is required by the Regulating Authority is deemed misinformation.
Here, LHO has passed, perhaps because of misinformation, and RHO has not yet taken a call, so LHO is entitled to change a call without other rectification. Moreover, I don't even see that any relevant unauthorized information has been transmitted to you, since partner realized their mistake before taking their next call, so I don't think that any further restriction on your bidding is necessary. Frankly, I don't think any restriction on your partner's bidding would be warranted even in other twists on this situation unless you said something before the end of the auction (or end of play, if your side ended up defending) to indicate that partner failed to alert.
By the by, LHO should have asked your partner about the meaning of your bid before taking a call. I suppose that your partner may have been thinking about her bid before being woken up to its meaning by LHO's question, but that is not unauthorized information -- that awakening was caused by your opponents, not you.
Third, what should her rebid have been after your 2C? Yes, I think 2NT is perfectly appropriate. Partner doesn't have 3-card heart support or a suit-oriented hand with rebiddable diamonds, and does have a club stopper. She clearly doesn't want to be in 3NT, but 2NT may well be the best spot.
Fourth, should she have opened in fourth seat? I think at match points this hand should be opened; the nice spade suit means that her side is not likely to "lose the auction" by allowing the opponents to back into a nice 2-level contract. As another answer indicates, 1S is a good choice for opening, but I do not go as far as he does and say that it's the only possible choice. 1D is also reasonable if she just treats this hand as a 12 count; her honor structure is nice and there are some nice spot cards. I think that choice is more defensible if your practice is to rebid 1NT with minimum balanced hands even with a four-card major, reserving 1S for more distributional hands.