If you have a link for the manufacturer or at least the product I'd might be able to dig it out of there, but beyond that your guess is as good as mine.

So I once again need to replace the Belt in my Symphony 200i 2016 (that's a new on I bought).

I found a replacement belt with the following marks on it "860 19.7 28", while I know the original part should read "860 19.7 28 T".

Do I really need the one with the "T" on it? what does the "T" mean?

There is no cost for the shot, but the outfits giving them will hit your insurance for some kind of charge if ya have it.
Far as how it's being done, up to about the middle of this month it's varied by state.  Mine, Washington, has notably lagged.  Political BS mostly, but some incompetence as well.

Personally I'm set for my first shot this week..annoyingly found that via an alternate route I could have done this last month, but as the French cest....La vie.

Almost apples and oranges there Sy.

Pretty sure the S'wing comes with a YUASA as an OEM.
SYMs do not.
Though some will argue that they're overpriced for the most part YUASA is well regarded.  I haven't really had any issues with 'em personally, & have only seen a couple grumble's bout 'em online.
The OEM batteries in Kymco's and SYMs as well as misc Chinese & other Taiwanese scoots seem a bit of a mixed bag, particularly in colder environments.

I am really surprised about the battery problems you descibe. I have had bikes for over sixty years, and scooters for the last eleven years,and this is the first time i have ever had a battery fail this way. I always maintained that, if rode regulary, a battery would hold it's charge, i must have been very lucky. My current Silverwing is six years old and i bought it new, it has 43,676 miles on it and i have never had the battery off to charge it. My previous Silverwing i had 40,000 miles on it and again never had the battery off to charge it. I will bear in mind what has been reported though, and check the battery condition often.

Yep, what Syd said.
They work until they don't.
The last one I replaced failed when I went to start the bike with a very audible <pop>.
Annoyingly it looked and spec'd OK until I put a load test on it which showed it was beyond hope.

Lowering your weight a bit does impact your ultimate potential top end, but most scoots are barely going to hit that.  By dropping it the weight you'll probably hit your actual top end, but the bike may seem kinda wrung out when you do.
Of course TANSTAAFL. 
It does increase your RPM, so you do impact gas mileage and likely to a trivial extent engine life as well.

Of these things you'll probably notice the MPG and RPMs the most.

I poked around just a tetch and found https://www.scooterdynasty.com/variatorrollerweightsfor200ilancescooters.aspx, & https://get2itparts.com/All-Parts/Transmission-Parts/Roller-Weights-Sliders/634325-CLUTCH-ROLLER-WEIGHTS-SET.

The latter is especially interesting since it cross references the OEM part number from the first and seems to note it as 13g & as the same weight used in the Lance 150s.

So judging by the 13 in their part number I'd guess at 13g as your starting weight.  Short of digging around more the easiest way to find out is to simply weigh one of your existing weights.

Far as how low to go.  Kinda depends on your end game.  I dropped both my HDs 1 gram. Sliders in one rollers in the other.  Works great.  2g would be a waste, especially if the drive train is the same one that was OK for the 150 series so it's already a bit bumped up in that regard anyway.

One final note on this.  If you move 'em too far you may start getting belt slip, possible breakage, clutch chatter etc.
That would bring you into the world of playing with clutches which can be fun, but is not something I've really gotten into; albeit I've fixed a couple with OEM stuff.

So to sum my thoughts up.
Weigh one of your weights.  If it's 13g try 12.  OEM clutch 'n stuff should be fine there.
If you really want to play with it you can get something like this (verify your size & order appropriately.  Seems to be 18x14 but I didn't do more than glance) https://www.scooterworks.com/Prima-Roller-Weight-Tuning-Kit-18x14-6g-to-17g-P1936.aspx

Heck Sy,
I'm with you for the most part, I move my lips when I read a multi-meter.
There are probably a couple folks left in this forum who are actually good at it, but I can follow basic directions so sites with instructions like those are a godsend.

Whatcha think of the SYM vs. your B400 ?

Huh.
If you're still running the original battery, & this hasn't happened before it might be time to get a new battery.

Cool Sy,
Is it easier to push again?

Over the last few days i noticed when i put my bike away in my Garage it was getting harder to push, i thought  maybe brakes binding, or sticking on.  On inspection the rear brake pads were getting pretty worn down, the front brake pads not as bad. I phoned the dealer his price for a full set of pads ( there are four pads on the front and two pads on the rear) was £89. I looked on ebay. I got the full set for £20.The pads arrived today so, i fitted the rear pads, after cleaning the brake caliper up. I cleaned the front caliper but decided the existing pads still had a lot of use in them after comparing them to the new pads. The pads i bought were Kyoto. I  have used this make of pads before and had no problems.