Magento – Upper limit for the weight in “Table Rates” shipping method

collectRates() is called for each shipping module.
It receives as parameter an object instance of Mage_Shipping_Model_Rate_Request. Let's call it $request.
Your collectRates() method should start like this:

if (!$this->getConfigFlag('active')) {
   return false;
} 

This means 'do nothing if the shipping method is disabled'.
Through the $request parameter you have access to all the products in the cart: $request->getAllItems().
You can get the total weight of the cart like this:

$request->getPackageWeight()

You can get the total number of products in the cart (qty):

$request->getPackageQty()

You can get the shipping address details as follows:

$request->getDestCountryId(); //for country code
$request->getDestRegionCode(); //for region id (if the country supports it
$request->getDestPostcode();//postal code
$request->getDestCity();//city
$request->getDestStreet();//street

These are the most important ones. For the additional info see the methods listed at the top of this class Mage_Shipping_Model_Rate_Request.
Based on this info you can calculate a price (or more prices) for delivery.
After you have the prices your collectRates method should return an instance of Mage_Shipping_Model_Rate_Result.
For this create an instance:

$result = Mage::getModel('shipping/rate_result');

Each result can support different methods for different prices. For each method you need to do the following:

$method = Mage::getModel('shipping/rate_result_method');//create a method instance
$method->setCarrier('some_code_here'); //set the carrier code (usually the code of your shipping method)
$method->setCarrierTitle($this->getConfigData('title'));//set the carrier title (usually rhe name of the shipping method)
$method->setMethod('some_method_code_here'); //a code for the shipping method. Example: 'express', 'fast_delivery'...It can also be the carrier code if you have only one method for the carrier
$method->setMethodTitle($this->getConfigData('name')); //the method title 
$method->setPrice(THE CALCULATED SHIPPING PRICE HERE);
$method->setCost(ADDITIONAL COST HERE); //if you don't have an additional cost (for handling for example) skip this line.
$result->append($method); //append the method to the result.

After finishing adding your methods simply return the result:

return $result;

Free Shipping. You can set it as the shopping cart rule and disable the "Free Shipping" method at all. It solves the first issue.

For the second issue, you probably need advanced shipping table rates module, there are a few of them at the magento connect.