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Difficult in summer, but for temp control in winter, I'd recommend getting a large builder's trug and an aquarium heater. Bucket in the trug with water, aquarium heater set to the temperature required dangling in the water and there you go. Summer you're a bit constrained, which is why I have a fridge/greenhouse heater/temp control unit for fermenting.
Charlie's right to an extent, but if you're getting acetaldehyde (which you don't want), it's likely to be happening at the fermenting stage, so making sure you pitch the right amount at the right temperature and controlling the primary fermentation is important. Sounds like you're leaving it long enough after reaching final gravity, so that's unlikely to be the cause, which is why I'm thinking it's during primary fermentation that it's happening. Your yeast isn't converting enough of the acetaldehyde by the sound of it, so it's either struggling to get going or it's not got the conditions it needs to do its bit efficiently enough to get rid of it.
With lager, it's such a difficult style to hide any brewing errors that anything sub-optimal is risky. So I really wouldn't do it without a fridge in this weather - you're almost asking for trouble. Even more so if the temperature is fluctuating all the time, which it probably is despite it being in the most stable room.
As for rehydrating, I wouldn't bother. I used to do it, but I found just bunging a packet in made little if no difference at all. And it's a faff, so you may as well stick with dry yeast directly in. Having said that, I've got more control over my fermentation temperature, so the yeast is operating in perfect conditions. You might find it gets things going quicker, which could help.
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ketsbaia
I haven't been doing anything to control the temperature other than keeping the fermenting bucket in the coolest and most temperature-stable room in the house. I have been following Charlie Papazian's book which seems to take a pretty laid back approach to the whole process and so I haven't been paying much attention to the finer details but perhaps I should start. I bottled it after 14 days as the specific gravity had been stable for 3 days at that point. Also used a pilsner style yeast that said it could tolerate higher temps - Papazian reckoned that fermenting a lager style at a higher temp would be fine if not optimal.
Do you mean leave it longer in the primary fermenter on the yeast cake, or rack it off for a while before bottling?
And thanks for the link, I've been using dried packet yeast directly into the wort so might try hydrating it first next time.