Sorry to hear you had a bad experience at immigration.
Truthfully though, there is absolutely no need for you to use a lawyer for your application. She hasn't been refused entry and as long as she doesn't overstay her 6-month stamp, then there's no reason why your application should be under any particular scrutiny.
Yes, she was questioned, but other than the bad attitude, the immigration officer was telling the truth - she was not obliged to let her back in again. Your fiance's first 6-month visa stamp became invalid the moment she left the UK for France, and so when you came back, she basically had to 'apply' for another visa to enter the UK again... same as the first time.
To be honest, given the issues she had when entering the UK the first time, I would not have recommended taking the trip to France, due to the possibility of her not being allowed back in again.
I know this sounds a bit pathetic and privileged, but we would quite like to pay an immigration lawyer, primarily just for definite info, and reassurance, and to keep the anxiety levels in check. If anyone is able to recommend somewhere with LGBT experience that would be much appreciated.
I would seriously not bother with a lawyer. Most immigration lawyers have very little experience with settlement visa applications filed outside the UK, and honestly, have no idea what they are talking about. We have seen several instances here on the forum where people have been given very bad, and very incorrect advice by lawyers for their visa applications, and had they taken their advice, their visas would have been refused.
We generally only recommend 2 lawyers here on UK-Y: Medivisas (based in London) and Laura Devine (offices in London and New York). They specialise in visa applications made in the US. However, they are expensive (hundreds of pounds, maybe more) and I wouldn't recommend using them unless she has serious criminal convictions, refused UK visas, refused entry to the UK, illegal working or overstaying in the UK.
LGBT couples are treated no differently than mixed-sex couples when it comes to visa applications so that will be irrelevant when it comes to applying
Thoughts also appreciated. I gather she will not be allowed another 6-month stay without a 6-month break first (?)
It's entirely up to the immigration officer at the border as to whether she can come back in as a visitor. There is no official '6 in 12 months' rule, but as she has already been questioned at the border and almost wasn't allowed in, I would leave it as long as possible (preferably at least 6 months) before she comes back in. She could also consider applying for a 6-month visitor visa in advance (this is multiple entry)
Also, I would suggest a shorter visit next time. The longer she wants to visit, the more likely she won't be allowed in (they will wonder why she can take 5-6 months off to visit the UK). A trip of a few weeks, maybe 2 months, will look much better than a trip of 5-6 months.
Essentially, what you're doing with this trip - seeing if you want to live together - is not actually allowed on a visitor visa, so if the immigration officer thinks she is here to try out living with you, they can refuse her entry. As we say here on the forum, there is no 'shacking up' visa - there's no visa that allows you to try out living together without getting married.