It's likely been a long time since your mil bf at least 20 years? So she could well be misremembering. Mothers and mil's don't always report honestly either.
My mum went on and on about how she bf all of us, it was only when I had dd and I was "still breastfeeding" at 8 WEEKS that I realised despite her claims she had actually only fed us for 6-7 weeks each. She thought feeding for longer than that was unnecessary!
I bf dd for 9-10 months and only stopped then because due to a medical issue my milk completely dried.
But dd and I were very poorly after the birth, dd was in scbu and was tube fed initially, then mix fed until we went home and then my fabulous midwife helped me transition her to fully bf. Which did include almost permanent feeding at one point around the 6-7 week Mark.
It's really not unusual and doesn't last very long, and yes it helps to establish supply.
I was never able to pump much yet I was able to provide enough for my heavy (long and skinny but heavy) baby to thrive.
If you want to keep bf and baby's gaining weight steadily (a little weight loss can occur but providing not too much and then quickly regained you're fine) then keep on as you are and you and baby will find the routine that's right for you both.
If you want to keep mix feeding that's ok too but it can affect supply in the early stages.
If you want to switch to FF totally again that's totally your choice but bear in mind after a certain point you'll no longer have the option to return to bf.
As per pps whatever you do SOMEBODY will criticise! So you may as well do what suits you and baby.
Where is the lactation consultant from?! She sounds utterly useless! I'd be asking about her credentials and experience to be honest! - unfortunately there are good and bad professionals in every sphere of expertise. And unfortunately bf expertise in this country is pretty crap still.
I had a health visitor appointment about 20 weeks in, we'd just moved house and dd was feeding more than usual which I made the mistake of mentioning. This hv made said maybe I wasn't producing enough milk and perhaps I should be thinking about moving to FF, that nutritionally it was pretty pointless bf "this late" luckily I was confident that this wasn't the case. As the conversation continued it turned out she was newly qualified, had no children of her own, had very little experience with children let alone babies and had done NO training regarding bf beyond the basic requirements of her qualification (which I suspect is pretty minimal!) why she became a hv I really don't know! She didn't even seem to like babies! I refused to see her again.
Try a different one, or sites like kelly mom or posting here...
She is talking nonsense! Bf babies don't gain weight fast, that's a good thing.