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Whether or not you get penalised or banned from social bookmarking sites will depend on how you use the automated software.  Many social bookmarking sites state in their terms and conditions that you're not allowed to automate the submission of links.  Others don't state that, but still reserve the right to ban accounts they feel are a misuse of their service.

In terms of whether search engines will penaise your sites - I think it depends on how many other valuable incoming links you already have.  If all your links come from social bookmarking sites you're not likely to see much of a boost to SERP rankings.

To avoid looking spammy it's generally advised to avoid using hyphens in domain names.  Also using hyphens means that a certain percentage of your visitors will type the name in incorrectly and end up on someone else's site (i.e. a competitor).

If possible go for a brandable name.

If your site deals predominantly with one particular country then go for the local ccTLD domain.

Otherwise, try other creative combinations of words.

Of course you could always try to purchase a keyword rich domain off someone else.

For this strategy to work effectively you need to use distinct domain names.

Keyword domains tend to rank well for those keywords, but it will take time to have a measurable effect.  The domains (minisites) will still need incoming links to do well - at minimum submit them to a couple of relevant directories, but the more time you can put into promoting them, the better.

You should probably have localized landing pages on your main domain too, but make sure the content isn't the same as that on the keyword domains.

Yes, that's a worthwhile strategy.  But don't just duplicate the content on each domain name - make sure there is unique and substantial content on each domain (don't just change the city name and keep the rest of the text the same).  Also, to get the benefits of doing this, don't duplicate content across the minisites and your main site.

Why update the main page's meta title monthly?  Is it to highlight special offers?  To stand out from the crowd?  To gather and improve CTR metrics?

Regular visitors will find your meta title familiar and wont need it to change often.  New visitors wont know what the old title was anyway.

Unless you can really track the success of meta description changes on a monthly basis I suggest you update it less frequently and concentrate on optimising other content.

Keep up the 4-times-per-week updates as long as you can continue to make the posts interesting and informative.  Be sure to tweet, bookmark and otherwise promote your blog posts - over time your shop will be found for all sorts of long tail search terms which should help to increase revenues.

To make your blogging activities easier I suggest you plan a few regular themes for posts - i.e. if you can write monthly posts on 4 regular topics that works out to 1 per week so you only have to think of 3 creative posts per week instead of 4.  By regular posts I mean things that keep the same theme each month and which are easy to write - e.g. things like some interesting statistics about the shop, monthly summary of your products in the news, etc.

Some things to check:

If your search results are being indexed is there only one URL per keyword combination?
Avoid showing the same content on multiple URLs - or restrict the SE bots to just one of them per keyword.

e.g. check you don't have a structure like this:

domain.name/search/location=xyz
domain.name/search/location=xyz&keyword=abc
domain.name/location/keyword

The same applies to the detail pages of your real estate listings.  I.e. don't let the SEs see content on all of these URLs:

domain.name/location/listing_id
domain.name/keyword/listing_id
domain.name/listing_id
domain.name/listing_slug

Rather than making duplicate content noindex I would prefer to redirect it to a common URL if possible.

To reduce duplicate content issues with client material, add extra info to the pages.  E.g. some things to try are:

• adding region/suburb info to search results pages
• pre-parse listings and extract key info on features/facilities/etc., then display that key info in a features box or something so that both the HTML and content differs a lot from client sites.