So, 2 questions that have been bothering me:
How does one incite sleep paralysis? I know it can be a scary experience, but it’s something that I’d like to try, and I know it can help with inducing lucid dreams.
If one uses a Reality Check within a dream and it doesn’t work, would one have to replace it? This has happened to me, and I did choose a different reality check to use, but I’m wondering if I should do it in the future.
I am not knowledgeable about the first question but I have some insights on the second one. Generally speaking different people have different reality checks that they prefer and that tend to have a higher success rate for everyone individually. So it’s not a bad idea to try different RCs. You probably want more than one anyway, it’s usually very handy to have at least a backup RC in case the first doesn’t give good results. Don’t overdo it though 
Which brings me to the more important point: it’s not so much about quantity or diversity of your RC suite but about the quality you execute them with. RCs go wrong from time to time, I think everyone has this experience. But they made it into your dreams already so that’s great. But if they don’t make you lucid (or take your lucidity away again after you already attained it) then you haven’t been doing them with the right mindset. The right mindset would be concisely to believe that it will fail (… to confirm reality, that is). From the onset your expectation should be that it’s a dream. If you’re doing the RC solely out of habit but without any ambition to actually make it fail then that’s a problem. And I’m mostly talking about your IWL RCs because you have better chance to influence how you are performing those.
To tackle this from a perspective of deeper understanding you should be aware that RCs are only a simplified tool. They don’t actually make you lucid. It’s your mental state that makes you lucid. It needs the right mixture of awareness, expectation and doubt (about reality). The RC is only a figurative act for channeling your mental state and (for me personally this is the biggest benefit) giving you reassurance, but that comes after you’re already lucid. Maybe you also achieved lucidity without RCs before, which goes to prove my point. If you haven’t yet, then I’m sure you eventually will as the message sinks in 