If you want to counteract the potential criticism aspect then ask her why she chose to ff - hopefully she will say that it's because it's what was best practice at the time.
And that's exactly the same as you're doing - just following the best advice available at the time. If you're feeling charitable you can even throw out a joke about how you reckon it will have changed again by the time you become a grandparent...
Hopefully she will agree - however if she doesn't, you (or even better, your dh) can go on the offensive and ask her to clarify that she doesn't want you to do the best you can for your baby - difficult one to argue against!
But should she persist, find out how much of what she did was what the doctors/advisers/baby guru of the day recommended and how much was what her mum or MIL did. If she says the former - why is she not extending that courtesy to you if she did it. If it's the latter - then what she is telling you to do is based on advice that is 50/60/70 (insert actual figure here!) years old and is that really what she wants for her new gc? And then remind her of some up to date research that has had big impacts - babies sleeping on their back and the reduction in cot death rate is a difficult one to argue with. (especially as the advice went from front to side before recommending back - fall in cot deaths seen both times - just because we have discovered a better way to do things doesn't mean that we won't discover something even better next day/week/year or that going with the best advice you had at the time but where the advice has been superseded meant you did something bad).
I desperately wanted to bf ds1 - after 4 days of no managing anything I called the la leche helpline and the lovely lady just said the important thing is that he feeds, not how. I gave him a formula carton that night, but he was able to drink it so that was great as he'd got very scrawny. Next day I started to express and did so for 3 months before (reluctantly) giving up.
Ds2 had maybe 4 or 5 formula feeds ever and just a handful of expressed feeds, we had a great midwife who really helped (unlike the first time who had put 10 minutes on the parking meter and was determined not to stay for a moment longer despite me giving her the details of my free parking space) and ds2 was finally reluctantly weaned off bf when he was about 3 years old.
I started with exactly the same intentions each time, but different outcomes. Having a pump is great. I also found that having one of mothercare's microwave sterilising bottles was great - a single bottle, you could assemble it in a certain way and then it just needed 90 seconds to zap in the microwave to sterilise. Saved on all the fuss and expense of a steriliser, but along with a couple of cartons of formula, it gave reassurance that if you did need a bottle for whatever reason that you had one and wouldn't be rushing to the supermarket at 5 minutes to closing and ending up with whatever they had regardless of cost or suitability. Also could be used to sterilise dummies if you ended up using them and it took loads of different teats.
I hope this doesn't sound like I'm promoting ff - I'm not. But if you have a pump then it's great to have a bottle to use with it. And to know before the tiredness/hormones/stress etc set in that you have flexibility of choice should you need it.
Just remember not to show the bottle and cartons to MIL so she doesn't start getting her hopes up 
Good luck - and remember to get dh to fight these battles so mil can't blame you!
(and sorry for any cross posts, I'm on my phone but been interrupted so taken a long time to hit post).