A lot of this ground has already been covered by others, but for clarification:
We're assuming that by "retirement visa" you mean an OA Long Stay visa. This will give you a year's permitted to stay stamp on entry and the visa is valid for a year from date of issue - not entry. You'll need to provide the Thai Embassy in London with financial evidence together with medical and police reports. The OP has said that he/she dosn't want the paperwork hassle, so move on to a O visa. (I appreciate that some members have got OA visas from countries other than their own, but they have tended to be resident in those. Unless the OP is in that situation, he/she will have to apply through the Embassy in London - if a Brit).
First off, there's no such thing as a multi-entry 90 day visa. You may be just describing it wrongly. O visas come as single entry - one stay of 90 days and visa valid for 90 days from date of issue - or multi-entry which allows you as many entries of 90 days as you want, as long as the visa remains valid. Visa will be valid for a year from date of issue.
1. Multi-entries are becoming more difficult to get but if you apply through a consulate in the UK, say Hull, (I assume you're a Brit), then as you're over 50 you'll only need to show that you have over 900 GBP a month income. Reason for wanting the visa is to check out retirement here. Take a look at their site -
http://www.thaiconsul-uk.com/downloads-and-visas.aspx - under additional evidence for Non O visas.
2. You can do as many 90 day "border runs" as you want as long as the visa is valid. Explained above. This process won't last indefinitely.
3. Explained above. Single or multi-entry O visas.
4. At the end of the visa's validity, that's it. You might get another multi-entry O in neighbouring countries, but they're becoming more difficult to get - a single entry won't be a problem. Embassies and consulates constantly change their requirements so you're best to check back here closer to the time and we can advise on the situation then.
If you want to stay on more permanently, then get an annual extension to your O visa from Imm here in Thailand/Hua Hin. The extension will be for a year and there's no need to exit the country, just report your address to Imm every 90 days. If you consider this option, again let us know and we'll give more precise details of the process. As you're over 50, it's really quite painless as long as you've got your finances - income or savings - sorted out.