I looked into Beer and immune issues quite a lot when ttcing. I do think you have a point with 'old' linings etc. But if you are doing a FET then they will not let you get an 'old' lining as its highly regulated.
I took steroids for several cycles with no sucess. I had an overactive immune system (e.g. I hadn't been ill for more than five years with anything and hangover illness doesn't count ahaha).
I had given up on doing it myself then got swine flu twice. After that I was ill for about 13 weeks straight with all manner of stupid things like colds and so on... you name it. I even got two colds almost back to back.
After that we did a disassterous surrogacy. However, in the back of my head I kept thinking 'humm I get ill now so maybe just maybe...'.
With the coaxing of the gals here I did another cycle with a megga proven donor and bam got pg.
Read Beer's 'Is my body baby friendly'. It sort of explains all you need to know. Once I'd read that I didn't feel the need to do further research.
I did ferret out a Beer doctor here in Australia and did phone consults with him. He was very clued.
When I did the last sucessful cycle I refused to take the steroids as they make you nutty. The doc said I didn't need to at that point.
For me I think it was our donor who was very proven and with the cycles she has done for others has nearly always produced children.
I gave my Beer book to my Re here in Australia.
Ask E&H about this stuff as she knows quite a lot, best, THK