ILR vs visa extension

[Desiro]

Hi all,
I am afraid I found this forum a bit late, but nevertheless, I will appreciate it if someone can help me out with advice. My Tier2 general RP expires on 14.7.20, this is a second RP(3 years) and the first 2-year T2 visa was valid from 29.05.15 and I first entered the UK on 14.06.15. It was not clear up to last week whether my employer would offer me the job in the foreseeable future, but now they are willing to extend my contract for another year. I started set(0) ILR application yesterday and realised that it is unlikely I will manage to pass Life in the UK test in time- no dates are available for booking yet. Also, it is not clear to me whether IELTS certificate from 2015 can be used for this application. Alternatively, I have a degree taught in English from Germany, but I guess Naric certification will take a while because they will have to contact University in Germany? I will therefore start an application for visa extension with CoS similar to what I did 3 years ago although I do realise that 6 weeks might not be enough for this.
I have several questions:
1. Can I submit two applications to HO, for Tier2 visa extension and later, when test centres open, for ILR application?
2. What is the optimal way to proceed in my situation so that I won't have to leave UK when my visa expires?
3. Given that visa is extended, are there any restrictions on when the application for ILR can be submitted, eg 2 months before RP expires or anything like that?
4. How is 5 year stay calculated- does it start from the day when visa is valid or the actual entry day to the UK?