First question: No, you don't record the "nominal" account balances for the period before the sale. The "purchase method" is currently used per GAAP. During the year, parent companies may keep an "investment in subsidiary" account should the subsidiary remain in existence. To prepare consolidated financial statements, eliminate the investment in subsidiary account against the assets and liabilities of the subsidiary with an offsetting entry to goodwill (if any) for the difference between the acquisition price and the net assets acquired. Note that the net income (loss) of the subsidiary allocable to the parent after acquisition is also recorded on the parent's books against the investment in subsidiary account and may be reclassified for financial statement purposes as part of the consolidation to reflect the subsidiary's actual revenue/expense accounts and minority interest. While the accounting principles remain the same across entities, the methods employed to accomplish the result vary widely and should be specific to your situation so as to maximize the efficiency, quality, and affordability of the entities' accounting processes. If you are personally unfamiliar with consolidating entries and methodology but these things are important (material) for the companies, I strongly suggest you obtain professional assistance.
Second question: If the subsidiary remains in existence, then it will typically continue to file its own tax return just as it normally would and report the full year's worth of activity on it. If the parent and subsidiary meet the criteria and make the election to do so, they may file as a consolidated group. The subsidiary, if still in existence as a separate legal entity, will be liable for filing its own tax return unless it is a disregarded entity for tax purposes (if disregarded, its activities after being acquired by the parent are simply reported on the parents' return).
These types of situations always present tax planning opportunities and pitfalls. For example, does the subsidiary have a net operating loss carryforward? Would it be preferable to file as a consolidated group? etc. etc. In general, before acquiring, the parent company should have built the tax implications into the purchase price (an NOL carryforward would generally increase the purchase price if the parent can make use of it, for example), so hopefully these sorts of things have been considered already. Unfortunately, asking this question makes it seem as though they haven't been. If that is the case, I again strongly suggest getting some professional assistance as soon as possible.