'Tone' is the innate tightness in a person's skeletal muscle. There are several different tests for it clinically (resistance to passive movement, reflexes, etc) but most of us have 'average' tone. It's a scale, and you can be low side of normal, or normal end of low, or really really high, etc Low tone results in a kind of floppier feel or look, and high tone is a tight, stiff feel and look. High tone is what can makes the limb of a person with cerebral palsy or stroke tighten up and not hang loosely when they are otherwise relaxed.

Many parents of babies with low tone describe they're babies are 'cuddly' because of the way they tend to relax into you when held. They also tend to like 'W sitting' which is with the thighs rolled all the way inward with knees bent right up so that their feet are next to their hips.

I'm not sure I would agree with carriers helping development in the same way as tummy time: different muscles work in different positions. Part of the benefit of tummy time is learning to lift and control the head and neck in that position, which uses the mucsle differently to the way they're used in sitting, and part of it is because they learn to take weight through the arms and shoulders, which obviously helps to build the strength they'll need to crawl.

What you are describing makes sense if she has never like being on her tummy - she screamed, so you naturally put her in positions she likes better! You could try to sneak it in without her realising - eg, you sit with legs out on the floor, and put her in a 'hands and knees position over your thigh/s or shins, with a favourite toy to distract her. Or, you lie on your back, and she lies on her tummy on your chest, so you are face to face, and her hands can push on your breastbone and collarbones, or put her hands on yours (still on your upper chest). You can bounce her gently like this, dance to songs, make funny faces, give kisses and pull away, etc. If you've carried her a lot and she loves being close to you, this might work really well to get her used to being on her tummy.

But it sounds as though her sitting and reaching are good, and she's started to get all the rolling sorted, too. If she is a bit behind, it sounds to me like she's moving in the right direction, and isn't too far behind. If she's sitting 'W', encourage her not too - should should sit with feet out in front of her, it because this position allows her to move into other positions more easily. Leaving toys further away, or holding them and tempting her just a bit more out of reach is good, and she'll not just lean out and grab the toy, but have to lean on the closer hand, lift her bottom off the floor and reach even further wth the other hand, to get to it. Make it a game, of course! That's the manouvre that leads them from sitting toward all fours.

As quite a few other Mums have said, it's not uncommon to be a bit behind developmentally, and most kids get there eventually, but a bit of extra help from physio, sooner rather than later, can work absolute wonders. Remember all of my advise, enev though i am a physio, is theoretical, and gives ideas based on general patterns, and not on your specific situation (my disclaimer!). I think it would be a good idea to get a referral to a developmental physio.