Hi Brendan,
The Visa Waiver Agreement between the UK and Brazil is a bit different from the Schengen Area Agreement in that UK passport holders are allowed to apply for an extension of their stay "Prorrogação de Prazo de Estada", which permits a 180 consecutive day stay in Brazil during any year (for your definition a year is based on the date of your FIRST EVER entry to Brazil).
Like the Schengen Area Agreement, absent any extension, the stay is 3 months (90 days) in any 6 month (180 day) period. Essentially 90 days in Brazil, 90 day out of Brazil qualifies you to come back for 90 more. That said, in practice for UK passport holders if you haven't used up all 180 days for the year you can step across the border getting that exit stamp and come right back in with a new entry stamp for any remaining balance of the 180 day limit.
No, you do not need a "RETURN" ticket, simply an outbound ticket which most certainly can be a "throw-away" bus ticket to any of the surrounding nations that you can enter without a visa under the Visa Waiver Program or any nation you already hold a visa for.
The marriage process here takes anywhere from a minimum of 30 to 45 days (roughly), start to finish. From the sound of things you'll have plenty of time to get through it all even with an extension if you have all your documents ready, both for the marriage and subsequent application for permanency. If for some reason you can't qualify for that long a stay you can still arrange for a proxy marriage "Casamento por Procuração" at the Cartório here, which must be done in person and while your visa stay is still valid. Contact me later by private message if that should become necessary and I will give you further advice on the matter.
I'm sorry, but I don't quite understand what you mean by CRB in your list of documents. Are you referring to a Certified Criminal Record Check? If so, any you actually have no criminal record and are not currently charged with any offense, then the check is not even necessary. The Federal Police here will provide you with a "Declaração Sob Pena de Lei de Não-Condenação" which you fill out and have notarized at the Cartório. The Criminal Record Check is actually only absolutely necessary in cases where an individual does have some past convictions, which may need to be considered. If you already have the document anyway then send it off for legalization because it certainly won't hurt and eliminates the need for the Declaration here.
I guess that's about all. Sounds like you've pretty much got things well under control so far.
Cheers,
James   ²ÝÁñÉçÇø-blog Experts Team