I am sorry if your question was not answered fully before. Here are what I believe are the missing answers:
1. Yes, you need to establish lawful residence before applying for citizenship. This is done through a long-term family visa (this applies even for countries such as the US whose citizens are exempt from short-term visas), then registering at the local town hall, obtaining a foreigner's registration card, and remaining in the country for the two years. The certificate of history of legal residence in Italy is issued by the police station, on the basis of your registration (which must be renewed yearly). They will inform you on how to obtain the tax stamp (basically it is an adjacent tax office, friends who did this tell me it is just a matter of sitting in several lines);
2. The wedding is not compulsory to take place in Italy. The two year period is for if you reside in Italy after the wedding, the three year period if you reside abroad after the wedding. However, the wedding, if performed abroad or at an embassy, must be recorded in the Italian civil status registry of your wife's residence. A non-recorded wedding is not recognized for this purpose;
3. Permanence of marriage bond is shown by a declaration made in person by the two of you that you have been living together ever since, and that you have not been separated in fact. If necessary, this can be given before a notary public, to be authenticated.
Finally, regarding the pension, the answer is negative. You will not receive one, because your age (52) if I am correct, means that at 55 (which is indeed the long-age pension limit) you will not have completed the required minimum number of years (currently over 10) of contributing into the system, to get a pension. Depending on where and how you contributed in the US, it might be possible to receive your US pension in Italy, but that is a US matter altogether. To obtain an Italian pension, you would need to, for example, work over 55 years, in a job which contributes to the national pension scheme, and make at least the minimum number of years for the particular job (for example, higher risk jobs have a reduced minimum) of contributions, and take pension later. It all depends on the job and sector of employment.
What you may be entitled to simply on the basis of age is a minimum old-age income. This is not a pension, but a kind of social-safety net provision of Italian law. However, it is not only based on age, but also on need. If you have savings and a US pension it is highly unlikely that it will apply for you, and in any case the amounts are very small (a few hundred euros monthly).
I hope my answers were helpful and look forward to your rating, which is essential to my activity.
Dr I L Vlad