Today was supposed to be my qualifier in Malaga, but I cancelled the trip a few weeks ago because I'm only just back running again because of injury. I hear this morning it's been cancelled because of flooding and the city centre is on red alert. Keep your feet dry Becca, stay safe!
For Italian races such as the Venice marathon, if you're a member of a club affiliated to your national body (e.g. English/Scottish Athletics), or directly a member of that body, you can present your membership card (or confirmation email, or something like that) instead of a medical certificate. Double check with the organisers, but this is generally the case with races up to marathon distance.
All that stuff above, I totally agree. In addition - I think your main consideration should be, can you fit in your weekly long run? This will be your key session every week, and the one you really cannot afford to miss. Eventually this could be three hours or more - can you fit this into your schedule? Bear in mind that it's not just the running time you need to think about but also fuelling up, recovery and possibly travelling to and from your running route, so in the later stages of your training you can expect it to take up a big chunk of your day. If you can work that into your schedule, I don't see why you shouldn't be able to have a good crack at a marathon. Having done a half already you're well ahead of many first timers at this stage.