there is no limit for entries yes but there is limit for staying in Thailand total in a year before becoming residents which is 6 month in a year. some officers enforce that and some are not, i also heard you can pay and get away with that...but after 6 month in a year you should not test your luck and get a long term visa like DTV or LTR. this guy was lucky to get away easily in Thailand otherwise people in europe can be in worst situation for the same situation.
I already have Tourist Visa, now i don't have to worry about anything for next 5 years unless something change, i can use visa on arrival also but that can change to ETA or something else...Less stress on other stuff is good for my work... in my line of work always have back plans.....
downloadable visa from the site is valid, make sure you print it out to show at the immigration, i wouldn't worry about resending in email because it will be the same as it is on there site....
Yes, once you get your 5 years DTV visa you can use it anytime after that, remember your visa will be valid from the time it issue and not from the day you will arrive in thailand....