1. It's highly unlikely they would move you to another base unless CIA downsizes or closes.
2. There will almost certainly be enough flying to mean forced unpaid leave is unnecessary. Usually they ask for volunteers for unpaid leave but it is very rarely forced on anyone.
3. I think you are right that sector pay on Italian domestic sectors is taxed in Italy rather than Ireland. My understanding (but stand to be corrected) is that this results in lower overall tax and therefore more money in your pocket. This was indeed calculated in the figures I originally gave. Expect an average of €6000 to €6500 per month across 12 months.
4. I've only ever known 5/4 even in the summer. Very occasionally they alter it but in 10 years at Ryanair I've never had mine altered over the summer period.
5. Sickness is fine. Ryanair say if you are sick for 1 or 2 days you don't need a doctor's certificate. If sick for 3 or more days you require some form of medical documentation but they do not insist on it if it's difficult for you to obtain. Overall, from my experience, the company these days is reasonable over sickness and accepts it happens. If you have an excessively high number of days then you are likely to be interviewed but initially in a relatively friendly manner.
6. For the type rating and any company travel (ie 'jumpseating' even though most of the time you can just take a passenger seat), you can take luggage and give it to the dispatcher and put it in the hold free of charge. They say one item of luggage for the hold is the maximum but in 99.99% of cases I am pretty sure you could put more in the hold if necessary.
Hope this helps. Good luck and enjoy yourself. It's a far better job and company than people on Pprune make out. Not perfect by any means but actually provides reliable, decent enough income, enjoyable flying and a stable roster.