Please comment - TTC

Hi, first cycle TTC - period varys 28-31 days - first day of last period 1st Dec - I use three apps they all said my fertile week was 11th-17th Dec. Would this change an be later if it was say a month where I had 31 day cycle?? Would it be similar fertile week??

I ordered ovulation tests but they only came Friday - did the test posted on here an people replied that I was very close - next day no lines at all multiple tests since no lines? Could it be I had just missed? Or ovu late even thou first test on the 15th said I was nearly? Sorry if these are silly questions I'm new to this!

BD on 12th 13th 16th 18th - is there any chance I could still conceive this cycle? Any tips? Xxxxxx