Nothing has denoted that the version available from github won't work on that version of PHP. As for out-of-stock saleability, it is primarily up to how your store is setup as to whether an out-of-stock specific attribute can be "oversold" or not. It has been designed to follow the overall settings of your store such that if your store does not allow selling product that are considered out-of-stock then the plugin will prevent the same at some point in the purchase process.
I have a few things to push to github for further improvements, but as with all plugins, I would suggest installing to a trial location before applying to your live store.
For those wondering about the improvements, well, have modified the stock reporting capabilities so that the listing in the dynamic dropdowns area is expected to only show items that can exist as defined in the admin (ie. Currently out-of-stock options are those that could theoretically exist based on the selections made on the attributes controller. On an upcoming github upload that "stock" display (absence of stock on identified/designated) will be added for dynamic dropdowns when using the default "filter" for multiple product.
Also, some javascript improvements have been made both on the catalog and admin sides. There are/may be some other improvements to be added depending on various factors, but trying to finish cleaning up the javascript before pushing it.
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