Be careful of the information you give here as it might cause major problème.What are you on about??
Stating an intention to apply for sponsorship does NOT make a person inadmissible.
Common-law relationships are NOT hard to prove. It is no different than a married app except submitting the proof of cohabitation.
Paying the fees ahead of time can most certainly changes things. It shows CBSA that a couple is serious about applying for PR and that they are following the process to legally live in Canada.
Inland sponsorship are always not the preferred option by IRCC. Declaring that you are coming as a visitor and intend to apply for inland sponsorship means you have a dual intentions. How do you then convince the officer that your intention is to visitor only and you will leave when your authorized stay ends? This is why most people with outland applications find it hard to get visitor's visa unless you are non-visa national, in which case you must declare dual intent. There are cases which applicants were turned back because they told Boarder officers that they are coming to apply for PR. There is à boarder issues TV channel that they show thèse cases.
Also paying for in land sponsorship fees in advance will not facilitate entry to Canada. This is relevant if you already have ongoing inland application and of proof of payment needs to be verified.
Also i think i wasn't clear here: I'm not saying common law partnerships are such difficult to prove. If you have monitored this forum for long, IRCC tends to request more documents from common law than married ones depending on the circumstance.
In a nuteshell, it's not necessary to debate over this.
My Advise:
Your partner should just enter as a visitor. You guys can travel together. Your partner's main intention should be just To visit you. As soon as you are in Canada you can begin your in land application together with OWP.
Also as Canuck said, your partner shouldn't bring many luggages as this will constitute their undeclared intention to remain in Canada.