(Apologies if I have missed relevant info in the meantime - kids have been freaking nuts.)
OK so centiles.
First of all we don't look at centiles for the first 2 weeks. Anything before that age is ignored, as it's about whether they are putting their birth weight back on after the expected drop. You just start from the first weight taken after 2 weeks old.
You didn't say anywhere what gestation she was born at, this is slightly relevant for birth centile, as in it can be calculated from that. However this is mainly for information as the birth centile isn't really relevant in terms of long term weight gain.
In the first few days, you had a drop by day 6 of 295g, which is less than 10% of her birthweight. About 8.5%. This is good. It's actually excellent considering your milk didn't come in until day 5/6! You then had a gain by day 9, which is great.
I noticed that your paediatrician said that she'd "only" put on 80g and was supposed to have put on 600g. That's not quite right, he should have started from the lowest weight point, not her birth weight. There is a very old, outdated, "one size fits all" but still used (particularly for some reason by paediatricians??) rule of thumb that babies are supposed to put on "about" 30g per day, but this is a very rough average, babies who start off small simply won't be putting on this amount, plus it won't literally be 30g per day in any case - it simply means that it takes babies a bit more than a month to put on 1kg. But in any case, if you had calculated it from her lowest point, then she had actually gained about 400g in less than three weeks. That's much closer to the 600g figure (wherever that came from) than 80g, and actually sounds about right for a baby on lower centiles.
You also noted quite correctly that later that same day at home on your scales the weight difference was huge. Of course different scales, different clothes, rounding errors, feeds may all have factored in but you can at least see that there is some difference expected in weighing. 1-200g could be neither here nor there, which is why it's very difficult to measure small expected gains like ~20g per day.
I've tried to plot on the charts and really it's much too small scale to get much from it, which probably means that centile charts aren't a helpful metric for weight gain over the period of a week or two. But she was sitting pretty much on the 9th by your post on the 7th, with your posts on the 11th and 13th these weights were in betweeb the 9th and 25th centile lines. The weight today puts her back to the 9th, which is generally considered OK. Babies do not grow in straight lines; it is normal for weight gain to bob over and under a line and this is classed as "following" that line. Or they may "bounce" between two lines - for example the 9th and 25th. These are normal and expected patterns of growth.
Lastly, you said that you do not want to go against medical advice without being trained, but your most up to date medical advice (from the LC) has not been to stop breastfeeding, and she has not (I assume?) asked you to weigh every 3 days, nor make decisions based on that. Your paediatrician has not requested another weigh-in yet either. So I would say that following medical advice would be to continue with the plan from the LC, in the absence of any other info from her.
In terms of weighing, if you really struggle with holding off for longer intervals, and find the fluctuation stressful, would weighed feeds be helpful for you, to reassure you that something is going in? This is how I did it:
Weigh baby before a feed you want to monitor, in the clothes and nappy they are wearing. Record weight. Be sure to have any dangling items (he had monitor etc wires on him, any dummy clips etc) on the scales, not hanging off.
Feed baby. Sometimes I'd do multiple feeds, if he was cluster feeding or feed/sleep/feeding.
Without changing any clothing, and before changing nappy, weigh again. Subtract earlier weight from this weight. The nappy part is important because it doesn't really matter if a wee is in their bladder or their nappy, if a poo is in their colon or the nappy, it needs to be counted the same as the only change you're measuring is the input ie the milk. You don't have to weigh immediately after a feed, you can let them sleep if you want to. It's just important not to change anything else weight wise so no clothing or nappy changes.