Heya,
Seems like on the whole you've at least thought through the redesign etc, so well done on that score. Here's my opinion on your questions though:
1. Leaving the XML file on-site for a short while won't do any harm - but make sure you have a new XML files containing the correct website structure going forward. Make sure your new structure has strong canonical tags so that the 301s are also recognised by their new URLs. As far as I know, the XML file doesn't help 'pass the juice', the 301's will.
The other consideration is to ensure you systematically look at external links to old pages and get them changed to point to new pages as the juice value of 301's diminishes over time.
2. You should only see a drop in your rankings if your new page content and titles become less relevant. So ensure your on-page optimisation is done well for your target keywords as soon after that page 'going live' as possible. If you drop in rankings, it may be temporary, or as you say with fresh content it may give it a boost. The issue is not your rankings, but ensuring relevance for your page. If you drop, compare your on-page optimisation to those of your competitors and see what they are doing differently.
If you do everything systematically/methodically and do it well then you should be fine.