The current owner, whoever that is (the bank or otherwise). The record owner of property is responsible for complying with the CC&Rs, including provisions requiring maintenance. If maintenance is inadequate (for example, if grass dies from lack of water) the CC&Rs may permit the imposition of fines, reimbursement assessments, or self help for expenses incurred by the association in performing the owner's maintenance duties.
At least some investigation into available equity should be undertaken. The type and cost of an asset investigation depends on many factors. They include the investigation cost, the proposed action (lien, small claims court, superior court foreclosure action, and nonjudicial foreclosures are typical options). At least minimal steps should be taken to identify the amount of senior encumbrances. In some cases, through short sales or other arrangements with senior lenders, the association may be able to obtain at least some of the unpaid assessments and collection costs due. In some cases it may be worthwhile to record a lien and/or pursue foreclosure even where equity is less than the value of the property (to protect recovery via a bankruptcy or to force a sale to obtain a paying owner or for other reasons).
It depends. Claims might be made under California's rent skimming statute (Civil Code §890) but likely depends on several factors. Violations of the rent skimming statutes could subject those involved to damage claims. There are ways of reducing the risks and the association's legal counsel should not be consulted.
Unfortunately for homeowner's associations, a lender foreclosure and sale of the property often results in no money for the association and the association is left to try to collect the money from the prior owner. In California, the rule of priorities is based on the "first in time is first in right" rule (there are a few exceptions to the rule but none that elevates the priority of the association's lien). Because the foreclosing lender recorded its trust deed prior to the association's lien, its interest in the property is senior to the association's interest. Once the lender forecloses and the property is sold at auction (or title reverts to the lender because there is no buyer), all encumbrances junior to the lender are extinguished. This includes the association's lien. (This scenario assumes there are no excess proceeds from the foreclosure and sale of the property.) Lenders, of course, would be much less willing to lend money to purchase property if they could not ensure that they would be first in line to take the collateral in the event of the borrower's default. Additionally, the lender is likely already selling or taking back the property at a loss.