I would agree with your selection except I would use Samarawickrama as wicket keeper and leave Chandimal free to focus upon his on the field captaincy. If Gunathilaka was selected to open instead of Samarawickrama then Mendis and Chandimal could share the wicket keeping duties when fielding.

I think playing the extra batsman at number 7 is important since batting has been their weak spot in the last 2 series. Hopefully Matthews will be able to contribute as a 4th pace bowler and if necessary De Silva can support Perera as a second spinner.

I agree. I don’t see any corroborating evidence that Chandimal is not an effective captain on the field – in his last full series against Pakistan he showed his capability in this respect. I think that Karunaratne is a good choice as VC and the aim here should be for him and Chandimal to work closely together on the field, not only tactically but also to keep the players highly motivated.

In the longer term I would like to see Jayawardene and Sangakarra appointed to run the selection committee and manage the team. They have the best insight into the game and the ability to get the best out of the players.

With Matthews and Gamage now
With Matthews and Gamage now both out for personal/injury reasons, Kusal Perera should keep his place at the top of the order with De Silva replacing Matthews instead.
Fernando should still replace Gamage and so the only real decision is whether to play two frontline spinners or add Rajitha as a fourth seamer. As an experienced Test allrounder, Dilruwan Perera will probably keep his place as an experienced member of the Test side and also for his batting contribution.

That’s just a fantasy X1 –
That’s just a fantasy X1 – they won’t drop Matthews, Lakmal or Herath.

De Silva will come into the side probably in place of Kusal Perera who is really an ODI specialist and is currently in the side because Karunaratne was injured and is not in the squad. He could open with Mendis or they might consider moving Dickwella up as an alternative opener.
Dilruwan Perera might be replaced by either Vandersay or Dananjaya since his 1 – 134 was less effective than Herath who returned 3 -119.
Gamage may also be replaced Fernando especially since Kumara could partner Lakmal in the opening attack with Fernando getting an opportunity as the third seamer.

NB: The pitch in St Lucia could favour the seam bowlers so another option might be to replace Dilruwan Perera with a fourth seamer by retaining Gamage and using De Silva as a back up second spinner to Herath.

Possible Team
1. Mendis
2. De Silva
3. Chandimal
4. Matthews
5. Silva
6. Dickwella
7. D. Perara or Dananjaya or Gamage
8. Herath
9. Lakmal
10. Kumara
11. Fernando

First choice X1 and squad of
First choice X1 and squad of 16 for ODI world Cup in 2019

1. U. Tharanga
2. S. Samarawickrama
3. K. Mendis
4. A. Matthews (VC)
5. D. De Silva
6. D. Chandimal (C)
7. K. Perara (WK)
8. D. Shanaka
9. M. Pushpakumara
10. S. Lakmal
11. S. Madushanka

Other Squad Members
12. N. Dickwella (WK)
13. A. Gunaratne
14. N. Pradeep
15. D. Chameera
16. A. Dananjaya

Three changes that I would
Three changes that I would make. Paranavithana in place of Thirimane, Vandersay for Perara and Asalanka for Shanaka. Otherwise a well balanced Test squad. First XI probably:
Karunaratne
Silva
Mendis
Chandimal
Matthews
Siriwardana
De Silva
Prasad
Herath
Chameera
Pradeep or Eranga