Spousal Visa Overstayers

I arrived in the UK in September 1999 as a student, I began studying, after 1 year, my solicitor messed up my visa application by not doing his job, (he was struck off by the Law Society due to negligence in many other cases), as a result, I became an overstayer, I did not return home after becoming aware of my status, I stayed due to reasons I won’t express here.
I submitted an application on the grounds of compassionate leave to remain in June 2016; it was rejected in March 2017. I met a girl mid 2017, she is a UK national, we fell in love and are now engaged to be married, I submitted a second application on the grounds I have a spouse, that too was rejected in April 2018 on the grounds we haven’t been cohabiting for over two years. Two months ago, I requested a copy of my passport from the Home Office to register our impending marriage (3 times in two months), they have yet to dignify my request with a response, I spoke to my solicitor last week Monday and he advised me to return to Jamaica, get married then apply for a spousal visa.
I have never been convicted of a crime, I have never been in trouble with the law, my fiancé works for a reputable investment bank, neither of us have ever been married, she owns her own home, we both have significant number of family members in the UK. My question is, would it be difficult to acquire a spousal visa after getting married abroad, would the fact I have been classed as an overstayer for more than a decade hamper my efforts in securing a spousal visa to return to the UK ?

In theory it should not impact your application from outside the UK. According to the Immigration rules, as far as I know, people without immigration status in the UK cannot marry a British citizen in the UK, so there is a requirement to leave and reentry.

As you are living in the UK for such a long time, there might be an Article 8 exercised as you have built your private life in the UK and, possibly, family life also. This is for your solicitor to look at your situation.

The relationships you mentioned are simply too short to build a family life, I would think. It seems you’ve made the mentioned applications by yourself (if it was your adviser – I would change the adviser) without proper grounds and this might impact the outcome of further applications as there is no consistency there and it looks like the applicant is desperate to stay in the UK no matter what. I doubt, Home Office will appreciate this.

Therefore, I would advise to involve immigration adviser or dedicate some time and do the job yourself full time to obtain a tangible result.

It's not clear if the Home Office have your passport or a copy of it - If it is proving too difficult to get it back, you can apply for a Jamaican passport from the consulate/embassy.

You seem to have very good chances of being granted a spouse visa from your home country, since your spouse meets the financial requirements. Just ensure that you have all your paperwork and documents in place before returning, as the application is now sent by the British partner to the Sheffield office for processing. So compile all your cohabitation documents, photos, shared bills, etc along with your completed form before returning to get married. The processing time can vary but allow a six-month window for a decision.

Your overstaying should have no bearing on the outcome of your spouse visa, as you will be returning voluntarily. It only becomes relevant again when you are at the British citizenship stage, if that is something you want to apply for.