The Tropical Fruit Forum

Lebmung, are you sure that you have Ichang papeda or Ichang Lemon or something else.
I have been growing three different clones of papeda and they all have leaves smelling like lawn grass with a hint  of parsley, very different from lemon.

Tedburn, yes, Sarawak is considered as a synonym of Taitian.

Millet, I am growing some varieties in pots that are kept in cold frame during the two winter months, but later are moved to conservatory.  Taitian is in flowers at the beginning of March and fruits begin to be edible in November.

I think Taitian is the best pomelo variety available in Europe.
It is quite sweet and juicy in 9 months after flower. In isolation it is completely seedless.

It will be difficult to keep greenhouse at around 0°C all over the winter. At temperatures higher  than 12C it starts vegetation and can be damaged by sudden temperature drop. 
Poncirus is hardy up to -24C; it should be completely resistant in your climate if you put your pot in ground close to greenhouse.

With HLB out there why add one more unnecessary obstacle for trees to overcome?
 If you are going to use a rootstock why not at least try Sugarbelle which has been reported to be tolerant of HLB?

PT is highly tolerant  and even partially resistant for HLB infection.

Most probably a Yuzu but need a leaf photo to be sure. Ichangensis is very heterogeneous, could also be  CRC3931  that is rather globular in shape


In France we have C.ichangensis SRA241 that has also round fruits

Where are you in Belgium? If you speak French, read recommendations on forum Agrumes-Passion, there are many people from Belgium that successfully growing citruses.
Basically, I guess your problems are in roots, pots should be very aerating, citrus roots outside soil need a lot of oxygen to withstand  diseases. Most of citruses from Sicily are delivered in heavy clay, because they were grown in fields. In majority of cases the vendors are washing this soil, but just add some light substrate outside. The water is easily draining but the clay is becoming dry. So you should wash this clay on delivery.
Than the easiest way is to use a cheap pine compost that is always available in supermarkets, check it before, it should still contain some granulation. If you want to use something else avoid "Citrus" substrates, majority of them are very bad. Instead, a good commercial substrate for potted plants with addition of equal volume of perlite  should be fine.
To assure aeration, some people here are perforating the bottom sides of plastic pots with holes of ~1 cm and the same distance between them. You should not put any drainage matter to the bottom. To fill the gaps between roots just use water  flow. 
For the fertilizer- most so called citrus fertilizers in Europe are not appropriate. The closest to ideal is Feriligene fertilizer for geraniums  18-6-12  with Mg and microelements.

Satsumas are highly nucellar and their flowers are rarely contain pollen.

Seedless Kishu (Mukaku kishu) is completely seedless due to the abortion of all embryos early in development, but the pollen is fertile and several valuable hybrids were produced in Japan, including Southern Yellow that inherited seedlessness. This could be a problem in your project, since you will probably need subsequent crosses to other varieties to achieve some degree  of sweetness and will be obliged to use only pollen.

I am in zone 6a. I have a Poncirus trifoilata that I planted in ground this spring. It is about 3 feet tall. Do I need to provide winter protection? If so, would I need to include a heater?

I guess  6a is at the limit of a long time survival of poncirus.
Since you have a long period of frozen ground, avoid winter sun, put some mulch around the stem.
Ideally,a passive heat protection like a box of styrofoam will help, I do not think that you need active heating.

Just above the first pair of true leaves.

I guess first of all  you should use only  "untreated after harvest " oranges, since most of Spanish citruses contain fungicides in their peel.
By  boiling pith with baking soda you can remove the bitterness even from poncirus peel.
http://www.eattheweeds.com/?s=Sour+orange
Also: http://tropicalfruitforum.com/index.php?topic=35316.0