Agnes knows that Jac would be furious if news of his new business deal leaked out and says to Michelle:

...Lladdai Jac fi pe byddai'r cyfan yn dod mas

Lladdai is the short form of Byddai'n lladd. Both forms mean 'he would kill'.
I'm sure you've noticed that the ending of lladdai, or lladde in Agnes's dialect, is the same as that of byddai.

Other examples are:

gwelai or byddai'n gweld, which mean 'would see'
bwytai or byddai'n bwyta, which mean 'would eat'
darllenai or byddai'n darllen, which mean 'would read'.
So listen out for lladdai - 'would kill'.

When Michelle tells Agnes that she hopes Jac's business will be a roaring success...
Agnes replies

Agnes - Gyda Jac wrth y llyw - dw i'n siwr bydd e.

Llyw here is spelt L-L-Y-W and is not to be confused with lliw, meaning colour, which is spelt L- L- I -W
Llyw with a Y has a number of meanings in Welsh. Some of them are:'rudder, helm, steering wheel of a car, handle bars of a bike'. Since all these meanings are to do with guiding or directing, it's easy to see how llyw can also be used in a figurative sense to mean 'leader or ruler.' So, 'Gyda Jac wrth y llyw' means 'with Jac at the helm'; in other words 'with Jac as manager'. It may interest you to know that Llywelyn, the last native prince of Wales, is referred to as Llywelyn y Llyw Olaf.

When we want to emphasise an adjective in English, we often precede it with an adverb, which ends in -LY. For example, exceptionally good, terribly expensive, awfully late and so on.

In Welsh the technique is a little different - 2 adjectives are used and linked by 'o'. When Agnes says that Jac's deal had gone through fantastically quickly, she uses the expression 'anhygoel o glou'.

Agnes - Gwnaeth yr holl beth ddigwydd yn glou iawn - yn dofe?
Jac - Do...
Agnes - Anhygoel o glou a dweud y gwir...

Anhygoel o glou means exceptionally quick
The most frequently used adjectives in this technique are andros, arbennig, hynod and ofnadwy.

Listen to some examples:

andros o fawr - exceptionally big
arbennig o dda - extraordinarily good
hynod o ddiddorol - especially interesting
ofnadwy o ddiflas - terribly boring